Some modems have a feature called adaptive dialing.
When they are told to dial a number using Tone dialing, they check to make sure that dialtone has gone away after dialing the first digit. If it has not, the modem assumes the phone line does not accept Tone dialing and so switches to Pulse. When dialing out from a PBX, there is almost always a secondary dialtone. In a situation like this, you need to tell the modem to expect the secondary dialtone. For example, to dial 9 for an outside line, and then 7654321, use ATDT9W7654321.