I'll have to test that, but I don't think I've ever had that happen. The data flow usually just pauses until your voice call ends, then it starts up again. I suspect it depends on if the downloading app has its own timeout timer.
Still better than nothing. Imagine two couples, with iPhone owning husbands and wives with WiFi tablets.
Wife 1: Honey, can you turn on your Verizon hotspot?
Husband 1: Sure, but if a call comes in, you'll have to wait, okay?
Wife 1: Sure.
Wife 2: Honey, can you turn on your ATT hotspot for me, too?
Husband 2: Uh, no.
Wife 2: But you're not talking, and you've said that your data works even if you are on the phone. So turn it on!
Husband 2: Uh, I can't.
Wife 2: You SOB. Why not?
Husband 3: My iPhone doesn't come with a free hotspot.
Wife 2: Grrrr.
Wife 1: I love you, baby.