Hi there! You are not the only one who has had this problem. I've had this very same problem and it has been extremely frustrating. My husband and I purchased our phones in December, and he does not experience this issue. Took it back to the store twice for phone resets because I couldn't detect a definitive pattern for the ringtone resets. I've read in various forums this has been an issue, but most seem to resolve it with either a reset, a software update, or moving the ringtones to internal memory vs. the SD card. I tried everything that made sense to correct the problem (apart from phone update, as there hasn't been one available) and was never able to resolve the issue. Took the phone back in to the store this evening and they were very kind and swapped it out with a new phone, no charge (I think I almost fell over when they did this). I'm totally thinking I had a "lemon" phone. I'll try to touch base again if I continue to have the same issues with the new equipment.
Good luck!
PS- I was having this issue with all ringtones defaulting to the "default" tone: paid ringtones, self created ringtones, downloaded ringtones, or even assigned pre-loaded ringtones. None of them would hold.