Well well my exact problem.
I dropped the phone, while i could hear others in normal mode I had some very loud noises during loud speaker call and people could not hear me in either times.
I sent it to a repair store!
It came up that the new introduced Mike in iPhone 4 was the problem.
They soldered it back and it was fine. Later on I find that my Wireless is reception very Poor (I have not tested GPS).
I think the repair store as probably you have left the G.P.S./WI-FI/Bluetooth antenna unscrewed or some connector to it.
I have been very wary to send it again as I doubt on the service(the did smth like this once).
I will send again but would need an opinion of what is missing first!!
Any body to help us is welcome!!