i got my entire family to switch from blackberry,symbian,ios and android to switch to lumias
my grandad has a 720 which had the same issue and i could not hear him properly and it sounded like the volumes levels increased and decreased every few seconds
anyways i figured this issue out soon when nokia care centre couldnt diagnose the problem
solution-change ur sim and get a precut sim from ur carrier
a full sim trimmed down to a microsim always has issues
in my family there are many 720s,820s and 920s and all were working fine except my grandads 720 which works perfectly now
hope i helped if so pls let us know
