Can't answer phone calls on macOS Mojave
Solution 1:
I had the same problem. The reason was that my MacBook Pro was connected to two networks, one via WiFi (the same one as my iPhone), and the other one via Ethernet directly to the router, bypassing the Access Point. Once I shut down the Ethernet, I was immediately able to answer phone calls on the Mac.
Solution 2:
I had the same issue since starting to use a VPN a couple of months ago - just switched the VPN off and retried and it's working fine - not an ideal fix, but gets around the problem for now...
This might fix your issue as well.