Dial up modems and Asterisk

How do I configure a dial up Modem to dial out through Asterisk?
Currently my dial up modems are set up with an extension on a ZAP channel just like our analog phones.
We are using Rhino Channel Banks, Asterisk v1.4.2.I have tested the lines and can dial out normally with a phone
but the modem says "remote party disconnected call, timeout looking for connection"
I tested the modem by plugging it directly into the trunk bypassing Asterisk and it connects to the remote server without problems.