I will recommend to take a look at wavecom cellular modems.
We use a nice modem here called WMOi3. It's made to placed
on a PCB.
http://www.multicap.be/Wavecom/P...dwmoi3.htm

Otherwise, you can take a look at the WMO2-model with
connector 15-DSUB, direct with RxD TxD RTS CTS and all,
- full-port.! even made for ordinary 'talk-mode'.