Verizon 3G CAN do it, but they turned it off so as not to 'cripple' the network. of course, you can if you're on wifi.
"Every phone on the network is using what is called the control channel. This always open channel is what the phone and the base station use to communicate. It is used to establish voice channels, data channels, initiate hand-offs between towers, text messaging, etc. So even while you are using a voice or data channel (1x for voice or EVDO for data), the control channel is still in use and still communicating with the tower. So say you are transferring data over EVDO, and someone calls you. The base station tells your phone that there is an incoming call. Your phone says 'ok', ends the EVDO session, tells the base station to close the EVDO channel and open a 1x channel for the phone call (assuming you answer the call). Or, say you are currently in a call and someone calls you. The base station tells your phone of the incoming call over the control channel. This is how call waiting or 3 way calling is supported."