You should get a better, more helpful, reply from the VM forum team on here. The VM telephone support typically are just reeling off answers from an on-screen script.
Other common features of the phantom calls are they go on for a very long or very short period of time or very exact periods of time e.g. multiple short calls of 1 second etc. or calls at unsocial hours in the early hours of the morning.
Do you know what time the call was placed?
Have you actually tried calling the number (during a free call time so as not to generate another charge) and see what it connects to (business, residential etc.)? You should then be able to confirm it has nothing to do with you.
Do you have anything else connected to the line which might generate a charge (security alarms, care alarms, call blockers etc.)? On a past topic a call blocker was ringing out and generating charges.
Wait for the forum team to advise when they get here.