OK, I misinterpreted your posting. It's not a matter of an 'alarm' message but rather a notification message in the lock screen. That's pretty much typical for apps to show notification messages in the lock screen window, so if that Voicemail app is a carrier installed app, you apparently have a carrier-locked phone with the added minus being since that app is presumably a pre-installed utility you won't be able to Uninstall it. At best you should still be able to Disable the app, but that could be a problem since that also stops the voicemail service to your carrier service account. If you do want to just Disable the app, you might need to check the Settings >> Apps menu and find the Default Apps option before the Disable button is usable. Open Default Apps and un-select the Voicemail app if it's in the list (Might not be). Instead of turning off the app, it might be better to just track down just what that notification message refers to. What does the notification message actually state when you swipe down from the Status bar?