Unfortunately the list of instructions (starting with Dial 901, press *, press 5 etc) didn't work. Here's what I said on the O2 forum;
'Thanks to Dave-O2 for the instructions; my voicemail from O2 was, and alas is, still stuck on my iPhone 14 (iOS 18.4.1) despite following them. However after executing, at least the blue dot on the voicemail screen has gone, as has the red dot on the home screen phone icon.
But the 'Network Provider Voicemail' text remains. All it is, is a list of computer Menu options, but nevertheless irritating.
Incidentally this voicemail message only appeared shortly after having renewed to an 18 month contract by phone; this seemed to trigger it.'
And several ideas from the Google Search link also prooved negative.
So how do I take the battery out ? Never done that. Don't see any easy way in; it's not the sim card compartment.