Yes, I tried restarting and completely rebooting the phone. I tried to go through voicemail and went through all the menus to find one that might let me reset the notifications. I even turned off notifications on the phone but it stubbornly refused to go away.
However, today I had another voicemail, listened to it and then deleted it and lo, the icon has now gone and it says Virgin in the place where the icon was.
So, thank you for trying to help but it looks like it has reset itself.
Helen