Garbage on the screen is usually a symptom of a baud rate mismatch.  I
would suggest you try experimenting with the baud rate in reflection. The
modem may be able to speed sense when you are offline giving it the dialing
commands, then shifting to a preset speed when it goes online. Also the
baud rate on the support link must match the port speed if the port on the
DTC is not set to speed sense. If you are getting a connection (modems go
online) then it probably is not a modem problem.