VM do only a very limited amount of customer communication by email; The online offers are a tiny subset of the total available; Outbound marketing is usually pushing a similar subset of what's on offer, and all outbound marketing is looking to sell packages at the price VM want.
The only way you'll find out the full range of what you can get, and haggle the price down is to phone up and select options for thinking of leaving (assuming you're out of any fixed term contract), and ask what their best offer is to keep you. Even then, if you don't do your research, using any price comparison website to see what Sky and BT are offering, then you'll be on the back foot. Your only leverage in getting a good price is a credible threat that you'll leave if VM won't offer you better value. Not quite the same as your circumstances, but adapt the ideas in this link to your needs.
If these calls are nuisance calls that you won't answer, why not register with the Telephone Preference Service, and also change your marketing preferences in My Virgin Media? And finally, you do know that the displayed number of an incoming call is trivially easy for criminals to change for scam call purposes, so you shouldn't use that as proof that the caller is who they claim to be.
