Thanks for your reply, Nat.
The voicemail notification has no details when I swipe down, and the "clear" is greyed out, so I can't tap on that.
I still have the issue of voicemail pin - I don't have any recollection of ever setting one up. Will I need it to access genuine voicemail messages?
It may be unrelated, but ever since I have had this mystery voicemail notification, my battery seems to only last half as long as usual...
Thanks again
Ragroll