This was a scam call. VM does not make unsolicited tech support calls to customers out of the blue.
You are very likely to get more calls either pretending to be VM or any number of other big-name tech firms (Amazon, Microsoft, BT etc.). They will all be scam calls with the aim of trying to commit fraud against you.
If you are receiving regular calls from unknown numbers, do not answer them. Let them ring out or go to an answering machine.
Consider fitting a call screening device such as trueCall
https://www.truecall.co.uk/category-s/116.htm
This screens calls (rather than trying to block numbers, which is an impossible task). Anyone calling your line gets a message telling them to sate who they are and press a specific key on their phone key pad. Only then does the call progress and make your phone ring. When you answer you hear the caller's recorded announcement. You can then decide whether to pick up or end the call without ever speaking to the caller.
The scammers' phone systems cannot usually complete the trueCall screening process and an individual scammer will not waste time trying as they know the person on the line has put in anti-scammer measures so will be scam aware.
The Met Police has a booklet on phone scams here
https://www.met.police.uk/SysSiteAssets/media/downloads/central/advice/fraud/met/little-booklet-of-phone-scams.pdf
and scams in general here
https://www.met.police.uk/SysSiteAssets/media/downloads/central/advice/fraud/met/the-little-book-of-big-scams.pdf
If you ever need/want to speak to VM, place and outbound call yourself to one of the recognised VM numbers 150 (free from a VM landline) or 0345 454 1111 (call charges may apply).
