Somehow it is dependent on operator, but how and why - I don't know!
The problem is that, sometimes, especially when CPU is busy (playing game for example) When incoming ring appears, sometimes default melody (nokia tune) plays instead of user-defined melody! It depends on operator, and CPU load, so on phones like 6630 it is rare situation, but on 6600 - it happens often. I've checked ALL series 60 phones, including SX1 from siemens and X700 from panasonic. Problem still there.
What we can do? Even installing FScaller and such apps won't help. Usually, custom defined melody starts to play, but after 1-2 seconds it is replaced by nokia tune.
Was tested on: 3650/3660/6600/7610/6630/SX1/SendoX/Panasonic X700.