Welcome to the forums, and let me start by saying I know exactly how you feel, I had a similar issue with my voicemail about a year ago, it was driving me mad. I'm sorry to hear that it's affecting your line as well.
I'd suggest calling your landline from a mobile phone, but not answering it, wait until the voicemail answers, then leave a test message, it doesn't need to be long, about 10 seconds.
Once you've done this, dial 1571 from your landline and listen to the message completely, then choose the option to delete the message. This should reset the line for you.
If you are still having an issue after doing this, then please let me know so I can look into it further for you.