I am not able to answer incoming phone calls on my Mac running macOS Mojave. The call notifications pops up both on my iPhone XS and my 2017 MacBook Pro, but when I go to answer the call on my Mac, it continues to ring on my phone.
I can make calls from the Mac just fine, but unable to answer incoming calls?
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.
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.
On my iPhone I have the Cellular data turned on and under the Cellular data tab under Settings I also have 'Allow calls on other devices' turned on. Then under 'Allow calls on' i have my iPad and my desktop computer turned on. When I receive a phone call all 3 devices ring and Facetime opens on my desktop. I get a panel allowing me to accept or cancel the phone call whether it is a Facetime call or not. If I click on accept, the phone call is answered, and I can talk to the caller.
Under Facetime preferences/Settings, I have all the boxes checked under the 'You can be reached for Facetime at:' settings. I don't know if this helps you but this works for me.
