It depends on your phone model, a good starting point would be a battery removal or if your device does not have a removable battery, a full restart (not sleep mode). If it still does not work, then it could be a hardware issue with the microphone, in which case you would need to take it in to a product care center for diagnosis. You can find a list of the closest product care center here http://www.telus.com/content/stores/en/find-a-store.jsp
@-uaj319: In addition to the video test, suggested by 'Xray' try these. Test(1). Go to sound >tap >ringtones >scan the list >locate the ring.Silent >if selected >de-selected. We don't know the kind of phone, you own, but the (Samsung.S4.S5) it has a silent ring. The Iphone I've no clue.
Test(2). Call your Vmail if the recording is audible mute button, malfunction is eliminated. Test(3) Ask a friend
to call you if Uheard his voice, mute button is ok. So try too 'Dimo-x' link to take it for
diagnosis. Finally search in 'Google' How to fix Mute-button issues' in (Iphone & SGxy.s4.s5) <ReadMe-soon> "v9"