Transfer custom ringtones to iPhone using a computer running Linux
Getting ringtones onto an iPhone is one of those things that's never been as easy as you'd expect.

Might be because Apple never thought it was important. Might be because Apple also sells ringtones in the iTunes store. Whatever the reason, fact is, there aren't a lot of ways to do it.

I don't know of any Linux way of getting them on the iPhone directly.

I believe that the iOS app GarageBand is the only app on the iPhone capable of adding a custom ringtone. Therefore, if you can get your ringtone from your Linux machine onto your iPhone (perhaps by emailing it to yourself and then using the "Share" feature on the iPhone to send it to GarageBand, that might work.

GarageBand is a very large app, but it's free, so worth checking out, I'd say.