Facing problems during calls on One Plus 8T

My mom has been facing this issue for quite some time, like sometimes there's no audio from the phone during calls, or you can't hear her from the other side, or sometimes both. This can't be reproduced every time but it happens frequently for both incoming and outgoing. Doesn't happen on whatsapp calls though. Anyone knows how to fix these input output issues if they are some software problems? Or should I go to the service center.