One of the areas where Microsoft’s Lync and Skype for Business products have had the most success is in replacing business phone systems (usually known as private branch exchange, or PBX, systems). This success has come about because Skype for Business has grown to offer a robust feature set that matches or exceeds what legacy PBX systems offer, combined with a thriving ecosystem of devices and add-on products and all the benefits that come from its integration with Windows, Active Directory, Exchange and SharePoint.

When Microsoft added Lync Online to the Office 365 product portfolio, it had a problem: customers who had bought the on-premises version of Lync would have to give up the ability to make and receive conventional phone calls through Lync Online. That forced customers to adopt one of a couple of different strategies: Use Office 365 only for presence, conferencing and instant messaging; deploy a hybrid environment and keep all the telephony features on-premises; or forgo Lync Online and stick with existing on-premises systems.

Why PSTN Calling Is a Challenge

The public switched telephone network (PSTN) was the Internet before there was an Internet: a globally networked telecommunications system, built mostly on open protocols, that was at first expensive and limited but soon became ubiquitously available throughout nearly the entire world. A great deal of careful design, engineering, and politicking went into its creation and operation throughout the world. All its complexity was largely hidden from users; people came to expect that they could dial a local, long-distance or international number, and have the call go through with reliable billing, decent sound quality, and access to value-added services such as emergency calling (911 in the US, 999 in the UK, etc.), Caller ID and so on. One of the early challenges Microsoft faced with its on-premises products was how to replicate those features, especially for emergency calling, but the company managed to overcome them through clever design and engineering.

The problem of how to enable a cloud-based service with the same capabilities was much more complex, however. Selling an on-premises product that interconnects to the local phone system in any given country is one thing; making a global service that works everywhere, honors local laws and regulations (such as India’s ban on using voice over IP to bypass long-distance call charges), and fits into the global infrastructure for billing and transferring calls across national boundaries is quite another.

When Microsoft bought Skype, it got a robust worldwide network of local PSTN points of presence (POPs), plus a lot of specialized expertise in PSTN interconnection. It’s taken a while for that knowledge to migrate over to Skype for Business, but now Microsoft is launching a preview program for what it’s calling “Cloud PBX with PSTN Calling.” This new Office 365 feature is intended to give customers a completely cloud-based replacement for their PBX systems. Users you enroll with this feature get their own PSTN phone number, which provides two benefits: It can be dialed from any phone (in which case it rings their Skype client), and it allows the Skype for Business client to be used to make outbound calls to regular phones. (Note that you’ll see this feature labeled as “Skype for Business Domestic and International Calling” in the Office 365 management portal.)

Don’t confuse this feature (which I’ll refer to as “PSTN calling”) with PSTN conferencing, a separate feature properly known as the Advanced Meetings Add-on for Skype for Business that lets users add dial-in audio conferencing to their Skype for Business Online meetings.

The Preview Disclaimer

As I write this in July 2015, PSTN calling is only available as a preview. And, unlike a Google preview, when new services are widely available to the world with “preview” stamped on the Web page, this is a true preview: It’s available only to a subset of Office 365 customers. You need an invitation code, and your tenant must be a commercial tenant in the United States. No government or academic tenants are supported, at least in the first wave of the preview. I expect Microsoft to widen the preview before PSTN calling launches as a feature, but the company hasn’t said anything public about a schedule for doing so, or for rolling the feature out to other countries. Because this feature is in preview right now, the user interface, enrollment process and feature set could change during the preview period. Keep that in mind as you read this.


Deploying PSTN calling for Skype for Business on-premises can be complicated, with a long list of requirements and prerequisites. On the other hand, PSTN calling in the cloud is much simpler. For the preview, your tenant must be located in the United States, and you must be willing to select and use phone numbers from a selected list of cities (including Memphis, Miami, Chicago, New Orleans, Houston, Austin and Dallas). Your Office 365 tenant must be on an Enterprise plan, and the users to whom you add Skype PSTN calling must have at least a Skype for Business Plan 2 license (which includes the E3 and E4 licenses). The service isn’t available for government or educational tenants right now. As with the PSTN conferencing preview, you can use either a cloud-only or hybrid Skype for Business tenant, but all the users you enable for PSTN calling must be cloud-only.

One significant issue to keep in mind is that Microsoft is explicitly not providing emergency calling through the 911 system during the preview. 911 calls don’t seem to be blocked, but they will not provide the location information required in the United States. And, even if the call goes through, unless you’re using a number from the city where you happen to be, that wouldn’t be helpful. Once the service is formally launched, I expect Microsoft to provide full support for emergency calling.

What You Get in the Preview

When you set up your tenant and enroll users, you get a pretty impressive list of features:

  • Each user gets a telephone number that can be used to receive incoming calls and make outgoing calls. You request a pool of numbers, then assign each user a number. However, you don’t get to pick specific numbers, and you cannot port your existing phone numbers to the service.
  • U.S. domestic calls are free.
  • International calls to most countries are free. There are 30 countries you currently can’t call at all (well, 29 countries, including Chad, Samoa, and Madagascar, plus the continent of Antarctica). The list of supported countries may change during the preview.
  • Many of the on-premises Skype for Business features are included, including team ring and the ability to redirect incoming calls from your Skype device to a mobile phone or voice mail.

As with the other Skype preview features, Microsoft hasn’t yet said anything public about the cost of the PSTN calling feature, when it will be available in the United States and what other countries will be supported.

Enrolling in the Preview

First, you’ll need to go to and request access to the preview. Once your request to join has been approved, Microsoft will email you a signup link. Sign in to the Office 365 portal, then paste the link into the same browser window and you’ll see a screen like the one below:

After clicking through the signup screen, you’ll get a receipt via email; then you’ll need to wait for your tenant to be provisioned. Microsoft says this can take up to an hour, although on my test tenant it took about 10 minutes. As soon as the provisioning process is complete, you’ll see “Skype for Business Domestic and international calling” show up in the licenses view in the Office 365 admin portal. At that point, you’re ready to start provisioning your tenant and users for calling. During the preview, Microsoft is granting 100 licenses for PSTN calling per tenant. There’s no way to change this number at present. However, once the feature moves to general availability, there’s a good chance Microsoft will be happy to sell you as many licenses as you like.

Provisioning PSTN Calling

Before your users can start making calls, you have to do two things: reserve one or more phone numbers, then assign one phone number to each user.

To reserve phone numbers, open the Skype for Business admin center using the “Skype for Business” link under the Admin category in the Office 365 admin portal, then select the “Skype voice” tab.

On the “Phone numbers” slice, you’ll see an icon of a telephone labeled “New numbers.” When you click it, you’ll get a simple form with three fields: the state and city where you want to reserve numbers and the quantity of numbers you want. Fill these out and click the “Add” button, and you’ll see a summary of the available numbers that looks like this:

Because each of the supported cities in the preview has a finite pool of available numbers, some of the cities listed in Microsoft’s documentation no longer have numbers available. For example, when I started writing this article there were numbers available in Miami, Florida and San Jose, Calif. A week later, they were all gone. (Somewhat inexplicably, “Indianapolis” appears in the state list. This is pretty obviously a simple coding mistake by someone who missed that day in elementary school social-studies class; I expect it to be fixed shortly after this article is published.)

You can de-select individual numbers using the checkbox next to each one. That’s all the control you have--you can’t request specific numbers or ranges at this point, although I imagine that Microsoft might add this as a premium feature in a future release because it would allow companies now using direct inward dialing (DID) with consecutive number ranges to have consecutive ranges when using Skype.

In my tests, I could only request a maximum of 200 numbers; asking for more than that (or more than the number available in the target city) gets you get a polite error message: “No available number for this tenant to reserve. Please wait and try again later.” This doesn’t clearly indicate whether the problem is that there are no available numbers (in which case, waiting might solve the problem as reserved but unused numbers are scavenged) or that you’ve requested more than the limit.

Once you’re happy with the presented list of numbers, click the “Finish” button. The number request window will close, and you’ll then see your new numbers in the “Phone numbers” tab. From this tab, you can see what numbers are in your pool, where they’re located, and whether they’re assigned to a user. The checkbox next to each number allows you to select numbers and delete them from your tenant, which returns them to the pool of available numbers. This view doesn’t show you which numbers are assigned to which users; for that, you need to switch to the “Voice users” tab.

Assigning licenses and numbers to users

As with many other operations in Office 365, there are multiple steps required to enable the PSTN calling feature for your users. First, each user needs the appropriate license assigned to his or her user account. You can do this in any of the usual ways: selecting the user and using the “Edit licenses” link in the user portal, opening the user properties and using the licenses tab there, or using Set-MsolUserLicense in PowerShell. You’ll see the “Skype for Business Domestic and International calling” license option in the admin portal. Select it and save the change to license the user.

Assigning a number to a user can be done in a couple of ways. First, all of the users who have licenses for the PSTN calling feature will show up in the “Voice users” tab, like this:

While it isn’t immediately obvious, clicking on the funnel icon allows you to filter the displayed users by showing all users, only those who have PSTN numbers assigned, or only those who have no number assigned. (Thanks to Microsoft’s Neil Deason for this tip!)

Selecting a user in this view causes an “Assign number” link to appear on the right-hand side of the window. You can also assign or change an assigned number by editing the user’s Skype properties, either through the “Edit Skype properties” link in the main user portal or from the Users tab in the Skype for Business admin center.

Licensing users and assigning a number to them sets several attributes on their user accounts in Azure Active Directory. The two most significant changes are that the lineUri attribute is set to a tel: URI containing their phone number, and the account is assigned the “Business Voice” voice policy. (This actually happens when you license the user.) You can verify this by using PowerShell to query for users who have that lineUri attribute set, like this:

Get-CsOnlineUser | Where {$_.LineURI –ne ""} `
  | ft DisplayName,LineURI,VoicePolicy


DisplayName    LineURI

-----------    -------

Paul Robichaux tel:+1504xxxxxxx

Michael Wilke  tel:+1832xxxxxxx

Scott Edwards  tel:+1832xxxxxxx

Ben Curry      tel:+1832xxxxxxx

Michael Pigott tel:+1832xxxxxxx

Brian Laws     tel:+1832xxxxxxx

Making and Receiving Calls

One of the best aspects of PSTN calling with Skype, whether on-premises or in the cloud, is how well it works within the client experience. Calls sent to your PSTN number appear in your Skype client just like any other incoming call. You can answer the call using the Skype client, redirect it to your mobile number or to voice mail, respond with an instant message, or ignore the call and let it ring until voice mail picks it up. When I called my Skype PSTN number, even the Mac Lync client noticed and gave me a call routing “toast,” just as it would with an on-prem PSTN deployment:

Speaking of voice mail: Remember that Skype for Business doesn’t provide its own voice mail services; that’s a feature provided by Exchange Unified Messaging. At present, Exchange UM isn’t supported for use with Skype PSTN calling, although I am optimistic that Microsoft will add support for this in a future release.

Making and Receiving Calls

In my tests, Skype for Business Online PSTN calling in the cloud worked exactly like PSTN calling with on-premises Skype for Business, except that there were no mediation servers or other on-premises servers or services to configure. It just worked, with minimal administrative effort on my part. Outbound calling worked equally well. The default voice policy applied to your preview PSTN users allows you to set up team calling, delegate access (for boss/admin calling, where a receptionist or assistant screens and initiates calls for someone else), and the ability to transfer calls from one user to another. Currently you can’t park calls, although you can place them on hold and then resume them.

I didn’t do extensive testing to see what kinds of calling restrictions, if any, Microsoft has embedded in the policies applied to outbound PSTN calls, although I’m sure that at some point they’ll be documented. It’s worth noting that, as with most other Office 365 services, the policy you get from Microsoft is the policy you get--you can’t modify it to enable or disable features or change settings. It would be technically straightforward for Microsoft to create multiple policy objects and use them to assign different feature sets to different groups of customers—perhaps one policy might allow domestic calling, then a different one might allow both domestic and international calling together—but I expect such changes, if any, to come after users have had access to the service for a while and Microsoft can assess usage patterns.


Overall, I’m pretty impressed with how seamlessly and well PSTN calling integrates with Skype for Business Online. In the past, Microsoft made what seemed like half-hearted efforts to offer cloud-based PSTN integration, relying on partners who didn’t seem to understand the business value such integration could bring to everyone involved. While this technology is only in preview, given the amount of demand for PSTN integration from cloud-only Office 365 customers, I expect it to be an immediate success once Microsoft makes it available in production. I also expect Microsoft to move quickly to try to drive adoption by offering its partners, customers, and internal sales and support teams incentives to put it into production as quickly as possible. So, if you’re interested in the feature set, I’d encourage you to sign up for the preview and start testing it to see whether it meets your needs.