Microphone not working during standard calls

When I make or receive a standard phone call, others cannot hear me, yet I can hear them clearly. Sometimes they can hear me briefly but it drops out after a minute or so and all they hear is silence.

My microphone works perfectly in all other applications: FaceTime, Viber, Skype, voice recording etc. I've had the internal microphone attachment replaced and I still have this problem during standard phone calls.

Please help, it's driving me crazy!