Hi Safetypin,
Thank you for your post, welcome to the Community Forums!
Sorry to hear you're getting a notification for a voicemail that isn't there. Just to confirm, is it a broken dial tone you're receiving to indicate a message or is it something else?
Have you tried using a new handset at all?
I'd recommend leaving yourself an actual voicemail from another phone, then opening it on the landline and hopefully it should disappear
Keep us posted
Beth