I have been trying and failing to turn off voicemail for a few weeks now. O2 CS just gave me the 1760 and couldn't assist further. I had deleted my guru app but signed in again to try and turn off VM... which it just told me it couldn't "error" etc. You need to delete your guru registration on settings in the app. Then you're free of it all and can successfully use 1750 and 1760 again. Hope this helps anyone having the same issue.
Had you have asked here we would have told you about tugo.
I have no affiliation whatsoever with O2 or any subsidiary companies. Comments posted are entirely of my own opinion. This is not Customer Service so we are unable to help with account specific issues.
Currently using: Pixel 7a (O2 & Lyca), One Plus 6 (Sfr), iPhone 12 Pro Max (Vodafone)