Hi Everyone. There is another (short) post regarding same issue. It's called "AW Cellular S3 Nike - Call Failed every time... A solution was posted on that string that worked for me as I can now answer calls on my watch. It states:
On your iPhone, go to Settings > Phone, and turn off Wi-Fi Calling and Calls on Other Devices.
In the Watch app on your iPhone, go to My Watch > General > Enable Handoff, and disable Handoff.
Restart both devices, then turn these features back on.
Here's more about Wi-Fi Calling and how calls are delivered to other devices:
Make and receive calls on your Mac, iPad, or iPod touch - Apple Support
Make a call with Wi-Fi Calling - Apple Support
Let me know if this works, test while connected and not connected to Wi-Fi.
Good luck.
Thanks for using the Apple Support Communities.