Q: After updating to iOS8, iPhone mutes itself during calls.

Hey AndrewA74

Check out the troubleshooting outlined below to resolve issues with the display not turning off when being close to your face. 

Phone: Hardware troubleshooting

http://support.apple.com/kb/TS2802

Screen won't lock or go black when answering calls

1. 1 Verify mute or speaker have not been activated, just prior to placing your ear over the receiver.
2. 2 While on a call hold your hand over the top third of the display, and verify that the screen is locked or goes to sleep.
3. Try turning iPhone off and then on again.

My issue is still not resolved. What do I do next?

Contact Apple Support.

Regards,

-Norm G.