Welcome to our forums and thank you for reaching out with your concerns. I'm really sorry that you've been having issues with your Virgin landline.
Usually, an intermittent dial tone would indicate that you have voicemails waiting, as you've already checked. I certainly wouldn't expect you to have this problem otherwise, we would need our 2nd line faults team to investigate this further.
The quickest and easiest way of liaising with them is to give us a call on 151 / 03454541111, as our frontline faults team can complete basic checks and then transfer you through with the relevant details. Have you tried calling and following the options for landline issues (2 > 4)?
Alternatively, we can clear security via private message and liaise with the team on your behalf. However, this may cause further delay to you receiving a resolution due to the nature of such a platform. If you'd rather we handle this for you, please drop me a private message with your account details and we'll take it from there.