Others can't hear me on phone calls!

Hi - I have an iPhone 4S that I purchased in October. I'm on AT&T. I haven't had any problems with it until now. The past few days, I've had issues with phone calls. I can sometimes hear the other person talking but they can't hear me and vice versa. Others can hear me when I put the phone on speaker but not when it's a regular phone call. Everything else works fine. Help!!