When someone calls my Android cellphone (a 408 area code number that's served by Google Fi), my Mac laptop (running 10.14.6 (Mojave)) "rings".
I do not have any Nokia or other phone-related software installed on the Mac.
The phone does not have the Mac paired with Bluetooth.
Indeed, I can turn off Bluetooth on the phone, and the Mac will still ring.
My Google Fi phone number is NOT a google voice number, and has no association with google voice.
The "Apple Message" program ("app", sigh) on the Mac does not have a record of my phone number ... and if I disable it, the Mac still rings. (I don't use the app, but it's there, so I thought I'd report on it :)
When the Mac "rings", there is NO other change on the desktop ... e.g., no window popping up with an "answer the phone" button or anything like that.
When the Mac "rings", NO new processes are created (according to "ps -ef" output), and none of the few that got CPU time before/during the ring look applicable to this feature/problem.
Although the phone and the Mac are on the same Wi-Fi, I can turn the phone's Wi-Fi off and the Mac will still ring.
I have no recollection of this happening prior to today (2020-10-31).