Seems like a very strange arrangement when the customer feels the need to pay £2.25 p.m. to stop his/her VM phone line running up unexplained international call charges!
Unexplained phantom calls get mentioned on here regularly. If you have a phone line from a traditional telephone wall socket (rather than via the hub) then a 'crossed line' is frequently put forward as an explanation (but phantom charges have also been mentioned for phone connections via the hub too).
If you don't have it already, would suggest you get full itemisation applied to the billing on your phone line. This should be FOC.
Quite often with these kinds of phantom calls, the cost ends up buried in the sub-50p (not itemised) default category and identifying the numbers called is very difficult to prove you didn't call them and have no connection with the numbers called.
Once full itemisation is in place, suggest you monitor you usage charges closely in 'My Virgin Media' and keep an eye on any charges to see if anything similar happens again.