if you are new to Virgin Mobile with a "new" mobile number it could be that your number has been "recycled".
I recently aquired a new pay as you go SIM direct from Three Mobile. Within the first week I was receiving text notifications from someone's Twitter feed, calls about an accident claim and texts about the latest offers from a fast food place at the other end of the country!
The best way I find to deal with all these is to just block them from your handset.
Many phone's have a call blocker built in but for my android device I downloaded a free app from the Play Store called "Calls Blacklist" (there are others but I find this simple and effective).
It's great for blocking all sorts of sales/marketing calls and texts.
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Virgin never had and do not sell or give out any info about you or your phone number to any non Virgin Media company you have a service with.
Sadly it possible that it just bad luck. A wide spread of mobile numbers are sent spam via a random number system (sadly becoming more common), if you then tried to stop one of there it flags that it's an in use number and then it can be sold on.
register the phone with the TPS to stop UK based calls,