iPhone 4S having audio issues - Other side hears his echo during call
I had this exact problem with a 4S until today. Here is what I did:

I installed Skype and Google Hangouts on the 4S to check whether this problem occurs with any calling application or just GSM. I made calls with Skype and Hangouts via WiFi and had no problem at all.

So I had a friend with a 4S with no such problems and I borrowed his SIM card for a test. I called his number on my 4S and had no echo problems! I put mine back and the echo returned. To make sure, I repeated this a couple of times. My SIM had echo, his no echo.

So I went to my provider's shop and requested a new SIM. Problem gone.

I hope this helps people who have tried all else with no results.  Why the SIM caused this is beyond me.