Because people really use the default ringers on cell phones. Hopefully this is different enough from the Game Music thread to exist on its own. Because you guys use non-game music for ringtones, right? Right?
So, I just switched over to using (1:24 on) from AC2 as my new ringtone, though ever since I got my new phone, I had been using (1:08 on) from Portal 2. For notification tones, right now I'm just using the default "DROOOOOOID" sound but previously I've used the sound from WoW and "Commander, you've received a new message at your private terminal" from Mass Effect 2.