Tethers fine. No problems with calls, only ever drops out in blackspots as you'd expect (Browns Lane being one of them).
I was nervous about 3 (I was one of the early movers to them years ago and the service was shite), but trialled my old iPhone with a 3 PAYG sim to see how it was for a couple of weeks before I took the plunge. Might be an option to see if you get the coverage you want at home / work etc?