IMO you did the right thing in stopping the call. Past posts on the forum have mentioned being asked for a banking sort code as part of 'security' checks (a dubious kind of check IMO). You can tell nothing from the incoming caller ID as it is easily faked to look like an alternative number.
VM have created a situation where their telesales calls are strikingly similar to scam calls (overseas call centres, unverifiable caller ID, mobile numbers being used for calls, pushy sales techniques/staff etc.). There have been past topics on the forum when customers have received similar calls (and the caller knew lots of info about the customer and their VM account) only to find later on that the call was fake.
If you want to renegotiate your VM package, the most reliable way to do that from a security POV is to place a call to VM yourself. Unfortunately, the deals on offer change rapidly so you might not get the same deal phoning back as you did from any earlier conversation with a genuine VM sales person who had phoned you.
