Callers can't hear me on iphone 6

I have an iphone 6 that has just started having a problem with phone calls.

When I call or other people call me then they can't hear me. To try and identify what the problem is I did the following:

tested the voice memo app and it records my voice and plays back OK

when on a call and change it to speakerphone then it works and they can hear me

facetime works fine and they can hear me

taking a video whether using the front or back camera works fine and the sound is heard

reset network settings - no difference

switched bluetooth on and off - no difference

upgraded the phone to the latest iOS update - no difference

shutdown all active apps and did a hard reboot of the phone - no difference

put my thumb over the proximity detector/sensor and now they can hear me.

Any ideas as to what is happening here???

Thanks