Playing a MIDI track involves synthesizing waveforms in real time, so I'm not sure what you're saying here. There have been plenty of phones that supported monophonic or mildly polyphonic ringtones without supporting full PCM, and plenty more that supported PCM samples but have limited space, kind of like old .MOD files.
I'm saying that they probably use a general midi sound bank, with sample sounds and limited control. You can only choose instruments from a predefined palette.
I doubt you could generate your own waveforms, apply your own custom effects and so on.
I never researched much into MIDI ringtones so I might be wrong.
