Useful links:
www.wavecom.com/showroom/bigframe.html
www.falcom.de
www.siemens.de (GSM cores)

I can tell you about Wavecom units (usual disclaimer: I'm
not linked in any way to wavecom)

Comms is done via serial using AT commands, these
are specified in GSM docs freely available at www.etsi.org
Very easy to work with.

I mentioned 'serial'. Yes, this is a RS-232 link but pinout
is not DB-9 as usual, but DB-15. You'll have to build the
cable. There is a switched regulator inside, you can apply
from 5 to 30VDC to the unit without damaging it.

GSM antenna: the new wavecoms have a SMA connector,
older had an FME connector

Internals: all baseband GSM processing is carried via an
ARM7 core made by VLSI (now part of Philips Semiconductor).

Regards,
Enrique.