Several people have reported problems with voice echo on their iPhone 4S during calls. The echo does not occur immediately but instead appears anywhere from 30 seconds-12 minutes into a call. It has been further reported that different headsets produce the same results.
Temporary Fix:
When experiencing the echo, turn the speaker function on and off to get rid of the echo.
Permanent Fix:
Let Apple know by filling out the following form:
http://www.apple.com/feedback/iphone.html