You and your students may not be aware, but not only do you not have to use your phone’s default ringtones, you also don’t need to BUY ringtones. The following links provide some tips on making your own ringtones using MIDI format or other format files. If the technology has left these instructions behind, or if these do not work for the phones you have, a search online should yield additional information.
And finally, the Wolfram team has created a free online ringtone generator which uses mathematical algorithms and randomizers to make completely unique ringtones in many different musical styles. Try it out!