I just checked my bill and over the last 6 months nearly every month I have been charged for multiple additional usage calls, but there is no physical phone connected to the landline?
Do you connect the phone to the VM hub or a normal telephone socket? If the latter, then a crossed connection on your line is sometimes put forward as an explanation. VM may offer to send a technician to investigate.
Are you able to see the numbers being charged for on your bill?
Past topics on this have included phantom calls either of identical durations or at specific times. Some have mentioned a whole range of different numbers. Some have listed calls in the middle of the night and in the early hours.
Do any of the calls involve premium rate numbers with 09 prefix or 08# prefix?
On some past topics, a simple analysis of the calls, the numbers and their patterns could show the calls billed for were spurious.