The Sony-Ericcson is the only composer(if you can call it that) I've seen on a phone. You would probably be best off using a midi sequencer on your computer and uploading to your phone.
I am looking for a cell phone with a polyphonic ringtone composer. I have seen the Sony Ericcson composer, but it just allows you to put pre-made riffs together to make a ring. I want to be able to compose multi-instrument rings note by note, and I want to be able to do it on the phone itself. Does such a phone exist?