SamKnows is still very much around and is the de-facto authority for speed tests as far as VM are concerned. If you use the VM post code checker you do not need to enter your phone number. Entering a Virgin Media phone number is just another means of them identifying which segment of the network you are on so they can tell you exactly which services are available to you because it can literally vary street by street.
You can use the post code checker and just put your post code and house number in. You won't get any phone calls or anything through the post. It is purely to tell you what services (and broadband speed) are available at the address you provided. Otherwise how else would they be able to tell you? I used it daily for months to see when 1gbit was available in my area. Use this link:
https://www.virginmedia.com/postcode-checker
As an fyi, if you put in an address (yours or anyone else's) which is already serviced by VM it asks you to login so you can see available upgrade options. If you don't want the faff and are after a quick glance of what is available regardless of price then just pick a different house number in the list which you know isn't currently serviced by VM. I know my neighbour is with BT so I always use their house number.