actually it is the opposite. AT&T uses voice and data on same network which allows both to be connected at the same time. Sprint and most others have it on different networks, which requires two separate antennas and connections to do both voice and data.
Myabe I'm remembering it wrong, but isn't AT&T the only broadband provider that uses separate networks for data and voice? The reason Sprint and Verizon will kick people off the service for violating TOS on their data services is heavy data usage can drag down a cell tower, crowding out voice service users.