ok some clear information.
1. SMS is sent via the paging channel so you can be on a voice call and still send, receive SMS
2. Data is carried over the same " timeslots " on TDMA and the same " code " on CDMA as the voice transmission, therefore it is not possible to have a data and voice session live at the same time.
3. Most networks air interface are set to prioritise the voice traffic so if you are on a data session ie sending email, the mobile network will page the BB to notify of the waiting phone call ( via the same paging channel used by the SMS ). when the BB responds to the waiting call the GPRS session is suspended and the voice call connected.
When the voice call clears down the GPRS is re started where it left off.