My now only newish mobile phone said, in its instructions, that to add your own ringtone, put the file in this particular folder on the phone, from which a settings command will allow you to transfer and designate it.
This requires plugging the phone by its USB port into a computer, and that's where the problem came in. When I did so, the phone wouldn't appear on the computer's device list. I'd had occasional problems on my computer in the past, so I tried it on two other Windows computers. Same result.
Finally decided to do something about this. I tried my all-purpose commercial computer gurus. They said there's probably an enabling or setup command somewhere on the phone, but they didn't know my model and didn't have time right then to search for it.
Tried the phone's help number. Out of our area of expertise, they said, and nothing in their manuals helped. Try one of our retail stores.
Retail store guy didn't know anything either. Even after I explained how I'd verified the problem he suspected that it was a problem of the computer, not the phone. I took leave to doubt this. Learned eventually this wasn't a real vendor retail store but a licensed one, so they're not even employees of the vendor. This was news to me; it was a dedicated outlet when I was last there, before the pandemic. Directed me to the nearest real store.
At the real store, they were busy but the person who took down my name for the customer waitlist, upon asking what I was there for and learning the answer decided to skip the waiting process and help me right away herself. After a little fumbling, located a command buried deep in settings called "USB storage." This was disabled; she enabled it. "That will probably work," she said, though lacking a computer on which it could be checked.
Took it home and it was. Now I have a ringtone I can remember is mine.