Why I cannot speak to why he wants a 2400 bps modem, I will say that yes, all modems even the latest 56k modems will negotiate down to 300 bps. That's one of the reason the training sequence takes as long as it does during call setup (it starts at the lowest baud rate and works its way up).
OT poster here - There is prior art that successfully used these two 2400 bps models. Even though the BBS will eventually use an ISDN PRI for PSTN access, folks dialing into it may be (unknowingly) using IP from their end
Training downward with an ever-changing SIP connection is known to be problematic. 2400 bps hopefully will provide a more reliable connection when SIP is involved.
Ok, in that case it's worth noting that many modems have AT commands which let you dictate the maximum connect rate. Such a feature would allow you to only receive incoming calls at 2400bps. Devin -- Devin J. Heitmueller http://www.devinheitmueller.com