iPhone voicemail notifications

Since moving to a provider without visual voicemail (a mistake...) a year ago, I have been plagued with ‘phantom’ voicemail notifications.

I don’t really use voicemail, so asked my network provider to deactivate my voicemail. Now, if I don’t answer, the phone just rings until the caller gives up.

Despite this, I still get ‘new voicemail’ banners about 8 or 10 times a day and it’s driving me mad!

I’ve tried every fix I can find online (which are mainly related to resolving the badge notifications issue, which is not present here), but nothing has been successful in stopping the notification banners.

Is anyone able to help? My only other solution seems to be to buy my way out of my existing contract with Three and return to EE (which has faultless visual voicemail and better coverage...)

Thanks in advance!