John gave you some excellent tips. It’s sad that someone is spoofing your father’s number like that and you have my sympathies for your loss. 🙁
One more thing that I would add is doing a factory reset on the cordless phone to erase any number in its directory and memory. Typically, in the Menu/Select option on the handset, there is an option to Delete All CID (caller ID) and saved phone numbers in the directory.
This way, if calls do come through again, maybe Mom won’t see Dad’s name as perhaps he/she saved an entry using more than his cell phone number associated with your Dad’s name.
Hope this helps you out,
P.S. If there is a voice mail greeting of your Dad that you would like to save, let me know. We can help you do that whereby we call the number, save the file on our studio computers, and send it to you. No charge, by the way.