That's not a given though. Firstly, you must have the option for marketing ticked in your customer portal, else you won't (or read that as shouldn't) get a call.
Secondly, VM are not ringing everyone back at the moment. They are doing selected ring backs; probably customers who accept £5 reduction. I have cancelled for example, and not had a ring back this time. Personally not bothered this time, as intended to walk way because of no escape clauses in their (unfair) contracts now when doing 2 price increases in an 18 month contract that legally you are held to, but don't know the price increase amounts when agreeing to the contract. Unless they offered me rolling contract or maximum of one year, I fully intend to be going elsewhere, and never coming back, and in notice period waiting for it to end.
Ideally, a company would want to keep existing customers AND take new customers. From the forums history though, there are some customers who expect ridiculous discounts way below competition pricing. If you stick around what competitors are charging for similar or better then, in my opinion, VM should negotiate to keep customers as their standard pricing is now out of control. (£75.07 for 500mbps BB Solo was mine!)
A new customer costs 5x or more than re-contracting an existing customer, they have to collect the kit (which they confirmed as doing), they have to check/test and refurbish that kit for other customers. Also loss of subscriptions like VM Stream. Then you have to get your new customers by offering £200 bill credits, free TVs, cases of wines and potentially if the new customer used topcashback or quidco cashback. Allied to friends and family who I am sure will also be leaving VM as I am their technical lead for advising what ISP and configuration of their kit 😉