It used to be that you could only disable by calling the unhelpful line (and a time before than when they wouldn't disable it).
That was a pure cynical play by the operators - getting termination revenue even if your phone wasn't answered.
If you have android, go into the dialler, then in options, settings "call settings" "call forwarding"
Set the various call-forwarding settings to "disabled"
That should be all you need to do. Verify by calling your mobe and you should find that after it times out you get a message that the target is unreachable, try again later.
Other systems will have similar settings. Your milage may vary,
This really should be switchable by calling the voicemail number too, not buried in an obscure menu:
related story: http://tech.slashdot.org/story/14/12/23/1948203/the-slow-death-of-voice-mail
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