drop, drop, drop!

My iPhone keeps dropping calls, even though I've got 4 and 5 bars of signal. What is up?!? Mike's drops calls sometimes too, but not nearly as often as mine. Is there individual variation in the phones? Have I got a lemon?

I've just made 3 calls that I first tried on mine and got dropped almost immediately, then used Mike's and was able to conduct the whole conversation. Grrrr