Hey mate,
Have you tried to record a voice memo to see if this is a microphone issue or, to do with calling in isolation?
Is your phone showing to have good signal strength?
Have you spoken to your network about their connectivity?
I’d suggest, resetting your network settings and if that doesn’t help, doing a backup and reset.
Failing that and, if the network are claiming good connection, I’d suggest contacting Nothing support https://nothing.tech/pages/contact-support
Enjoy the device, they’re great phone!