log in to your account then click on the "Plan and device" tab.
This takes you to a page "My tariffs and devices" & "Manage my services" with Voicemail is there.
Turn it off by moving the "switch" in your account.(Click the other side of the rectangle box).
After changing the settings, allow time for your account to update then reboot your phone.
Or give customer services a call (free on 789 from your mobile) and they will be able to turn it off for you.
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