You can definitely do it with bluetooth. You need to put it in the right folder, and occasionally rename the file.
On my razr I would put it in the voice memos folder I think it was. And then a renamed the .pcq I think it was. To do this you have to go into the Tools > Folder Options, then go to advanced and change the option so that you can see common file types. That will change the filename from "Sun's Gonna Rise" to "Sun's Gonna Rise.mp3". It lets you see the file format.
Once you put the .mp3 on your phone, change it to .qcp. Go into your phone, under sounds you should see your voice memos as well. Since you tricked your phone into thinking you just made a voice memo, you can set that as your ringtone.
I know that seems like a lot of work, but some phones have the ability to add custom ringtones locked out. So the only way to get them is to pay. this is a workaround I found for Razr's, not sure how it works for others.
There are a number of ways to do it. Search google for vx5400 verizon ringtone hack and you should find your answer.
Most ringtones, despite having different file extensions, are low quality mp3s. On the Motorola RZR and the derivatives (KRZR, etc.) you only need to access the filesystem over USB, copy the file, set the file permission to locked, then restart the phone. Very simple stuff, but you should still look it up.
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