Hi marios146,
We understand that you're experiencing audio issues with your Apple Watch, and we're happy to take a closer look. So we better understand, is your Apple Watch a cellular model, or GPS-Only? If it's GPS-Only, how far away is the iPhone when the problem happens? Have you ever been able to use the call feature on the Apple Watch successfully?
When you get a chance, please take a look at the resource below, and confirm that the Apple Watch functions are working as expected.
Answer phone calls on Apple Watch
If you swipe up on the Control Center during the call, and then tap on the AirPlay icon, does it show the Apple Watch as the audio output?
Connect Apple Watch to Bluetooth headphones or speakers
We look forward to learning the results!