Most modern cell phones use MIDI ringtones. The T610/616 is no exception.
However, MIDI isn't like an MP3 file. You can't just convert from MP3 to MIDI... think of a MIDI like sheet notation for a musical instrument (or several). The computer/instrument reads the MIDI file, and takes those instructions to figure out which sounds to play & when. It's not a true audio file, just instructions on how to make the sounds properly.
So, if you want a MIDI of a specific song for your ringtone, someone would have to create a MIDI file from scratch. Do some searches online, and you may find it... there's a surprising number of people making MIDIs out of every kind of song you can think of.