If you rceive:

Error 629: the data link has been terminated by the remote machine

when your multilink ISDN client tries to connect to a RAS server, it may be because the RAS server does not have multilink enabled.

The first channel connects at 64Kbps, but the second channel does not come up.

On the RAS Server, use Control Panel / Network / Services / Remote Access Service / Properties to select the ISDN adapter. Click Network and click Enable Multilink.

Shutdown and restart the RAS server.