Hi @Ruggiechick Welcome to the forum. How long have you had the phone? If less than 28 days you can return it if its faulty and you would need to ring O2 to get it swapped http://www.o2.co.uk/contactus
If longer than this time then please get in touch with Samsung directly. (Better to deal with these than O2 repairers) http://www.samsung.com/uk/support/
If this is happening on 2 phones it could be a network issue. Check your postcode here
Also check if any case or screen protector is covering the microphone.
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