Does the modem actually pick up the line?
Turn the speaker on in the modem and listen for the dial-tone.
Does it capture the dial-tone?
After a couple of seconds does it then dial the number?
Do you hear the other modem pick-up?

Do they hand-shake?
If it fails here, tough to say whats going on at the OS level of either side.
Could be a bad password, or even login in name.

Or give up trying?
If all that happens, then you have the modems set to incompatible set-ups.