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Recently had a new screen fitted and now when I'm on phone to someone when they speak to me there's an echo on their end for about 30 seconds that I can't hear. Could this be the new screen causing it or is it my phone case?
Hi @Superdingle, sorry to hear about this. 
Was the repair made at an authorized service centre and did you start to experience this directly after the repair or was it after a few days / weeks?
If it occurred directly after the repair, it does sound like it is related to the actual repair and not the network / software in the device.
You can however still test the microphone outside of a call and see if there is any change. To do that, go to Settings > Support > tap the middle icon at the bottom of the screen called Tests > Microphone, can you hear the echo when the recording is being played?
Let me know how it goes!