Yep many people have mentioned this regaridng the phone.
If you also type in on google c902 speaker problems there are many people on there also having simular issues.
You will need to get it repaired at a SE center, Carphone warehouse repair center, phoning o2 or taking it in to a o2 shop.
Welcome to the o2 forum as well
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Use iPhone XS Max dual sim, SE, Honor 10, IPad Pro 11, Apple Watch 4 LTE, 100+ Mpbs Barrier and a Record Player.