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, bumper or screen protector that you may be using on your Apple Watch, especially 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.
- Be sure to follow Apple's cleaning instructions.
- Cleaning your Apple Watch - Apple Support
- Important handling information
Restart both your iPhone and your Apple Watch, turning both devices off together, then restarting your iPhone first:
- Restart your iPhone, iPad, or iPod touch - Apple Support
- How to restart your Apple Watch - Apple Support
If the issue persists, next try unpairing and re-pairing your Apple Watch again:
- The Watch app on your iPhone takes a backup of your watch automatically when unpairing via the app.
- Initially choose to Restore from Backup during set up (most data and settings will be restored, with some exceptions such as Apple Pay cards and passcode).
- If the issue persists, next try unpairing and setting your Apple Watch up as a new watch.
- Unpair your Apple Watch and iPhone - Apple Support
- Set up your Apple Watch - Apple Support
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: