So basically iphones are rubbish, if you just get an Android phone then the GPS works just fine in flight safe mode.
I often forget to turn mine off, worst case is the annoying electronic noise on the intercom, can be quite embarrassing if it does that while you're making a call to ATC.
As for jamming the masts, I don't think it creates a massive problem from the low levels of a few thousand feet, after all you could be up a hill and be that high, my understanding is that it's a problem when well in excess of 10,000' and you end up in a "white zone" where you can connect to multiple masts at once which the system wasn't designed for. I although I also hear that the more modern systems are fixing this, pretty sure we'll all be able to make phone calls on passenger flights in the not too distant future.