Typical GSM current profile
I used some generic modules, so here are some values to work with:


voice call: 250 - 300 mA (unactive unused bursts: 20 - 50 mA)
GPRS (2Tx, 3Rx slots): 340 - 410 mA


This assumes having a matched antenna. The current consumption itself depends on the transmitted power and is thus dependend on the network. If you have a highly unmatched antenna, a voice call can draw up to 2500 mA.