We have no specific information available on the subject, however the process should be quite straightforward.
You need to configure the receiving modem in auto answer mode. The modem manual should give you the AT commands to use to do this.
Then, simply dial into it from the other end.
The only questionable issue is whether it is possible to operate the modem without the CTS/RTS handshake signals. The controller uses/provides none of them. So on the modem that is near the controller, you may need to pull up or down these pins to the appropriate levels.
During communication, the RTS/CTS are also used to control data flow. If you are using a 28K modem to transmit/receive at 9600 bauds (needed by the controller), you should normally never run in a situation where the PC or controller would be asked to wait before sending data.