This issue crops up regularly on the forum. There are some possible explanations for it.
Do you receive the VM landline service via a conventional telephone wall socket or via a connection from the back of the VM hub (the new VM 21CV service)?
A common explanation for a conventional wall socket connection is a crossed line in the street. VM will usually offer to send a technician to check this but often with the caveat of a £99 non-fault charge possibly being applied if no fault is found. In a past topic on the forum, the customer discovered a neighbour had been inadvertently connected to their line and was running up unexpected charges.
Errors also seem to be possible via a hub connection but have not been explained in public on the forum AFAIK. Some of these have resulted in large unexplained charges for customers.
Have you tried plugging in a phone to see if your landline is actually still working in your home (if your line is not working, you cannot be making calls from it)? If it works, have you tried dialling out to your mobile and seeing if the caller ID from your landline matches the one you are being billed for? Have you tried dialling your landline number you are being billed for to see if anyone picks up, which might indicate a crossed connection? You can do these tests during any free time in your phone package to avoid any further charges being applied.
One of the VM forum team should pick this up and be able to investigate this for you. They usually reply within a week or so.