Several times now I have had my assigned ringtones just vanish. Sometimes when they call I get NO ringtone and sometimes others go to my default ringtone. Why does it keep changing on me!?
Are these custom ringtones that you've installed, or the built in ones? If they're custom, did you put them on the phone's internal memory, or a removable SD card? If they're on a removable SD, try copying them to internal memory. That will probably fix the problem.