very simple, do it online on lebara.ch in 1 min.
pls long in to your www.lebara.ch
account [NOTE : first time user might need to setup online account, a free service for every Lebara user]
> click "MyLebara" (on top side of page)
> click "Profile & Settings" (on top side of page, below MyLebara icon)
> click "Your service settings" (on left hand side of page)
> trun "Voicemail" to off (with your mouse/cursor)
> click "save settings"
> check/re-confirm if "Voicemail" is showing off now ?