It differs Tristan because it's basically a compressed wav. And yes you can have REAL sounds on your phone, although you have to be careful which song or sample you want to use as you can tell the quality is poor.
On my GD87 I have some MMF files. You have to doctor them by using Windows Recorder., but the best format available for the GD87 is 16 bit mono@8,000hz.
Not great as I'm sure you've worked out. But it's good for voies etc. I have my message set as Barney burping out of the Simpsons, it sounds hilarious!
Not sure if the T610 supports mmf, although the tones sound great on that anyway. Suppose you've been to
www.myt610.com ???