I posted this a couple of months ago but can't find the link now. O2's Gateway's do not appear to have the MIME type for ".cod" files added to their UK Gateways. Any ringtone URL's with .cod cannot be viewed however a .midi extension allows a ringtone to be downlaoded to a 7210/3510/i succesfully.
I've given up trying to explain this O2 now!! All other UK networks work OK with the Nokia .cod extension!
Andy - ringtone.net