Yes I have done this, have you?

If you use the ringer maker that came with your phone, the resulting file will be a pdb. once you sync the file, it will be included in the list under your phone prefs, incoming calls.

edit: the ringer maker takes mp3s and turns them into sounds for the phone.

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My sordid past:

Kyocera 6035, Kyocera 7135, Samsung i700, iPAQ 6315, iPAQ hx4705, Motorola a630, BB 7100t, iPAQ rx3115, JAM 850, BB 8700g, Samsung i320 (just for fun), imate KJAM, TMO SDA, MDA, TMO Wing