Short call durations and an exact number of minutes per call has been mentioned on here many times before as a feature of 'phantom' calls (as well as calls being made at all hours of the day and night).
Does your VM landline connect via the hub or a conventional telephone wall socket. For the latter a 'crossed line' is usually put forward as a possible explanation.
One of the VM forum team should reply here shortly to advise on the process of investigating and, hopefully, refunding you.