I have an Iphone 4S that is not functioning correctly. When I make a phone call I cannot hear the other person talking and when the iphone is ringing to the other person. It works okay with headphones, using speaker and Siri works. I did recently replace the back cover of the Iphone, therefore not sure if that has caused the problem. I did restore the software, but that did no good. What hardware issue or part of the Iphone should I check 1st. Thanks for any help.