Hi
When making and receiving calls using Apple Watch, keep in mind that Apple Watch supports using compatible Bluetooth headphones that are paired to the watch for listening to music that has been synced to your Apple Watch, but not for making or receiving phone calls.
The following steps may help:
Check that you are following the instructions here when making or receiving a call on your Apple Watch:
Unpair your iPhone from a Bluetooth headset, if one is presently paired to it.
Remove any protective case or film that you may be using on your Apple Watch if it is blocking the speaker port (and/or microphone port):
Clean your Apple Watch, paying particular attention to the microphone and speaker ports, in case they have become blocked by debris.
Restart both your iPhone and your Apple Watch, turning both devices off together, then restarting your iPhone first:
If the issue persists, next try unpairing and re-pairing your Apple Watch:
If the issue then still persists, I suggest that you contact Apple Support (remote diagnostics and/or mail-in service may be available, if required), make a Genius Bar reservation or visit an Apple Authorised Service Provider for assistance: