If Dialer is saying "initializing modem" quickly followed by "connection failed," then you may need to patch /usr/photon/bin/phdialer.
What's happening is that before sending the modem initialization string and the phone number to dial, phdialer sends an "ATZ" command. The problem is that some modems do a complete reset when they get ATZ and discard everything else that followed it.
To fix phdialer, using a hex editor, change 41 54 5A 00 to read 41 54 00 00.