I think if you're getting data on wifi, then you can receive calls still. If you're on the cell network, I'm guessing you stop getting data and the phone switches over to receiving the call but not sure

verified with a friend this weekend who's using Verizon with his iphone 4s... when you get a call while using data on cell network, the data access stops and the phone call takes precedence (you'll still receive the call). if you try to switch to an app or browser for data while on a call, you get a message saying it won't work