Boris suggests we look at this GSM/GPRS and GSM modem.
A modem GSM & GPRS with SIM900 or SIM908 module. Allows you to create data connections on the GSM network through a standard USB interface. The cellular modems, particularly USB-stick ones, are now at very affordable prices, however they’re limited: they are explicitly designed for Internet connections, so you cannot use it as a normal modem and so implement, for example, a point to point data communications with them. The GSM modems that allow this are quite rare, and so we create and offer you one: it is a device for PC with an USB interface with “voice” functions: there is a jack for a speaker and one for microphone.