It's getting harder to connect through the modems as people migrate to new phone service. Even if the hand-off to the PBX is POTS or PRI, most telcom services use VoIP on the back end.

If you have some way to listen to the line, such as turning up the modem speaker or connecting a muted phone to the line, you should hear something that sounds like continuous white noise when the modems connect. If you just hear a series of different types of beeps, or beeps mixed with white noise, then the modems are having trouble negotiating.