GPRS devices are not as straightforward as you may think. There are in fact 3 different classes of device.

Class A:

Class A terminals have 2 transceivers which allow them to send / receive data and voice at the same time. This class of device takes full advantage of GPRS and GSM. You can be taking a call and receiving data all at the same time.

Class B:

Class B devices can send / receive data or voice but not both at the same time. Generally if you are using GPRS and you receive a voice call you will get an option to answer the call or carry on.

Class C:

This device only allows one means of connectivity. An example would be a GPRS PCMCIA card in a laptop.