You can specify which to be a priority ... data or voice. Thats what we did to our company line. Last time it was prioritize for data transfer and we kept getting missed call so we change the priority to voice. When you are using data and a call came in, it will stop the data transfer. The data will resume back after you end the call. This is the disadvantage of EDGE, it's not duplex in data and voice.

In TS case, if you're using digi + EDGE on iPhone 3G, try disabling 3G in setting and use the GSM band instead of HSDPA.