Cellular calling doesn't yet work reliably. Any improvement may only be temporary.
Make sure you've followed the instructions in this support article. Note that both iCloud and FaceTime on the Mac must be signed in to the same Apple ID; those are two separate steps. Also note that the Mac and the phone must be connected to the same Wi-Fi network. Ethernet won't work. Trying to pair the phone to the Mac as a Bluetooth device won't work either.
No Continuity feature will work if either of the devices is connected to a VPN.
If cellular calling still doesn't work, sign out of FaceTime and sign back in. You may have to do the same on the phone. The email address that you use as your Apple ID must be able to receive FaceTime calls.