This subject gets regular mentions on the forum, often from customers who pay for a landline service but never actually use it or even have a phone plugged in.
For customers connecting via a regular telephone wall socket, the usual explanation is the possibility of a crossed line in the street cabinet. The usual VM offer is to send a technician to investigate but with the condition that a £99 charge may be applied if no fault is found.
Customers connecting the landline phone via the VM hub will not have a simple crossed line but do seem to experience phantom charges (sometimes for large amounts of money in phantom charges).
Public information on the forum about the causes/resolution is rarely made available as the issue is usually dealt with via private message.
You can try some simple tests yourself which may help you and VM decide what to do next.
Have you tried ringing your allocated landline number to see what happens? If someone answers, and you have no phone plugged in yourself, then some crossed line type of explanation may apply.
If you can borrow a phone (or buy a cheap one to test with) have you plugged in a phone to see if your line is actually active? Can you get dial tone? Can you currently dial in/out from the landline and, if you can, what caller ID number is being presented for your line? Does it match the one you are being billed for? To avoid incurring any further charges you may want to do any testing within any free time on your landline package.
The final explanation, which sometimes features, is that another device (such as house alarm, medical alarm etc.) connected to the line is dialling out unexpectedly and running up charges. This is only relevant if you have any other equipment connected to the landline.
One of the VM forum team will pick this up and advise further. They usually reply within a week on here.