I am not communication engneering but it because the edge/GPRS/3g is using the gsm bandwitdh. This also cause then surfing,u use it to transmit data not voice.somehow now u can only do 1 thing at a time like answer call or surf. Basicly u can mix the 2 line type but only 1 active at a time. But u should able recieve call when in edge/3g.

Also apple is a company who only release the app when thy think is stable. Therefore we need wait LOL