How to lower a modem's speed
 

I am trying to lower the speed of a U.S. Robotics 56k V.92 USB faxmodem to V.21 300bps speed, in order to connect to another modem over this radio link I've made. I already have a way of hooking up a dialup modem to the radios, and I am able to get both modems to send the initial handshaking sounds, but the bandwidth limitation of the radios is making V.92, 90 and 34 connections impossible. I think they should be able to connect at a lower rate like 300-1200bps though. The only way I can test this, however is by giving it a shot.

Does anyone know how to do this?