Welcome to Android Central! Which phone? If you make a call with speakerphone, does the other party complain they can't hear you? If so, then the secondary mic at the top of the phone is probably blocked or faulty. Check to make sure the mic opening isn't clogged with lint or blocked by a poorly designed case. If it isn't, then the mic is probably faulty -- bring it to a repair shop, or contact the manufacturer if it's still under warranty.