This is a loose guide here, but if you tell use which phone you have we can be more specific.
If hey have already been transferred to the phone, go to your phone settings, then your sounds/notifications. Choose phone calls and when you tap ringtones you should be given a list of what is available on the phone, including the BT files. If you don't see them, or the phone doesn't see them, you might have to use a file manager or connect the phone via USB to a PC and then move the files from the BT folder to the sounds/ringtones folder.
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