Pop into your local O2 shop and get a free SIM swap first to see if it helps.
Also try using loudspeaker just to check it's not the way you are holding the phone or if something is covering the mic.
Check with the local network you are using to see if there are any faults or manually select a different network to try.
It won't be the mike it will possibly be a problem with the speaker but if it happens in other phones then it's the network I would think rather than the sim card.
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