We have found 3 possible solutions to your problem.
Before you start this troubleshooting guide you need to check one important thing:
Does your Apple Watch ring at incoming calls?
Yes
No
Why do we ask this question?
It's important to find out if your Apple Watch rings when there's an incoming call. If your Apple Watch doesn't ring at incoming calls, you need to solve this problem first. When the problem is solved, you will most likely also be able to hear the message tone.