As stated previously, the Calls appear only in one place, in the Recent Calls List on the handset. For it to have been sent to the handset and to not actually be a call (this is different from withheld numbers etc.), I surmise this list is probably provided by the IP Telephony servers.
As stated by myself and others it has not appeared before moving (in my case a BT DEC Phone system comprising an answer phone (no messages left) and 5 handsets) to Hub 5 and switching from Virgin Media landline to IP Telephony.
The fact that it occurs on multiple users, seems to be related IP networking, and that Virgin Media state it cannot be seen at "their end" (not sure if these means the servers or admin application?) would suggest the possibility of a man-in-the-middle brute force network attack against a (computer generated) list of phone numbers. That, more than the simple annoyance of the repeated calls, would be the main worry (if it is undetected thus far, what damage is it doing and where within Virgin Media systems and therefore, potentially, their customers)? This is why I flagged it as relevant to the Cyber security Team.
