It happens during facetime calls as well so carrier can be ruled out.
I also get this, it happens during the following:
Mobile calls
O2 Wifi calling
VoIP calls using Bria
Doesn't happen al the time, toggling the speaker on/off clears it
Audio during Music and Movie playback is unaffected.
Update: Been on to Apple and as I got it from O2 on Saturday they said to just return it and get a replacement. Doing a full factory restore and update via iTunes just to make sure its not an issue with the shipping build and then off to O2 I guess :/
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