Midi Ringtones

I’ve been toying the idea of composing MIDI ringtones. Are there special requirements for this? I’m all ignorant, I don’t own a MIDI-enabled cellphone (I don’t even use music ringtones at all!), so cellphones have the full GM set? Do I just provide the MIDI files per se or I have to convert them?

Most Cellphones can play GM/GS Midifiles nowadays. I own a k700i by sony ericsson and an old Siemens MC60 which play most midis without problem. I found out that it’s best not to use XG-Stuff but I know there are several midi-formats available. The simpler the file is, the better.