In most cases there will be a hand-off from EDGE to a voice call (depending on the carrier) and the call will take precedence over your data connection. However this doesn't always happen and at times callers will get kicked to your Voice Mail if you're using EDGE data. Unfortunately short of UMTS/HSDPA (aka "3G" in GSM terms) there is no way to run data and voice traffic concurrently over a GSM network