A connection via a wall socket does lend itself more easily to a 'crossed line'.
The pattern that is often repeated on here is that the customer does not use the landline and has no phone plugged in. At some point a fault occurs, or the line is disconnected, but the customer has no idea of this as the line is not in use and never checked or tested. The faulty/disconnected line starts generating call costs or registering calls to 999 etc.
You are now in a somewhat adversarial situation with VM who will be keen to recoup call costs from you.
Prior to the VM visit you should test the line and see if you have dial tone and can make calls in and out. The line may be totally dead or may be part working.
Without doing any verification yourself, you are placing total trust in whatever judgement the VM tech comes to as you won't know if the line was working or not prior to the visit.
I no longer have a landline but I keep a very simple gondola-type handset available for whenever I am asked to help anyone else with phone testing.
Something like this
https://www.amazon.co.uk/VTech-CD1100-Trimstyle-compatible-mountable/dp/B0CCDG5F7D
sub-£10 is suitable. You could test with that.
You should also log in to the VM hub and confirm that telephony is disabled on there.
If you do that before the VM tech comes you will then be in a position to compare with whatever the VM tech tells you the problem is.
While you continue to pay for a VM landline, you should occasionally plug a phone into the line and test it is working and you can call in and out.
Some of the past 'phantom call charge' topics on here have dragged on and on for months and have involved £100s of pounds in call charges and hours wasted in comm's with VM to try to resolve so it is in your own interests really to keep an eye on the operation of your VM landline, even if you intend to make no use of it at all.
