The complete message is:

Signing into SIP Communications Service failed because the service is temporarily unavailable. Please try again later.

This problem will occur if the domain that the Live Communications Server is configured to host does not match the domain portion of the Session Initiation Protocol (SIP) Universal Resource Identifier (URI) that is assigned to the user.

If the Live Communications Server is configured to host the Corporate.JSIINC.COM domain but the SIP URI references JSIINC.COM, you will experience this behavior.