You'll need to look and see what diagnostics are available from the Ascend. There could be problems at the following areas and there should be diagnostics to help you at least narrow it down. Once you narrow it down that will guide further troubleshooting
Layer 1 - Is there a problem with the ISDN line itself. Unless you know for sure how SBC tested it, and for how long you can't take their word as fact.
If your Terminal Adaptor (TA) has two POTS ports try dialing from one port to the other and leave it up for half and hour and see if the connection stays up and seems to be working. You can also dial to time and weather and listen for a while and see if you hear dropouts or other problems
Layer 3 - We know there is no main incompatibility here or you could not dial at once. However at Layer 3 you'll be able to tell if the call(s) is/are ebing dropped. If so see if the Ascend allows you to examine the Cause code. Not only should the cause code give some indication, but if you can getr the first octet value it will tell you what portion of the network dropped the call and issued the cause code.
Layer 4 - There may be some problem with the higher level LAN protocols.
I hope this gets you started.