I get this with my 6280. It's been happening for ages now, although went through a phase of not happening. Sometimes turning it off and on works (although usually not), and other times when someone does leave a voicemail and I then delete that one, it goes off. It's just annoying as you can't then tell when there's new voicemail waiting for you (well, not on Orange anyway). Other problems seem to be occurring too, also very sporadically, namely, my phone just turning itself off and then on again, usually mid-call, and also sometimes half my phone book disappears so if someone calls and you have their number stored, it will only flash up the number and not the name - so, no good for caller ID then. No good for texts either.
Should really send it back but don't know if this means that I will lose some stuff off my current phone, although most of the stuff I want to keep is on the memory card anyway.
Procrastination eh?!