its plain cold calling
i had a similar call just a few weeks ago .... also claiming that they were from virgin and that they could offer me a better deal as my contract was about to expire .... virgin had apparently partnered with o2 to offer customers a great deal (at least thats what the guy tried to tell me) and they could offer me a better contract on o2
i immediatley became suspicious as if it had been a genuine call from virgin they would have known that i had already renewed my contract just a few days earlier, so i started asking questions as to who they were and it turned out that they had no connection with virgin whatsoever .... they were just an independent company selling o2 contracts on a commission basis ... i asked how they knew my contract was nearly up and he told me he didn't actually know that and it was just one of numerous scripted ways of getting people to listen.
Don't be scammed by these type of calls, virgin mobile will be able tp prove to you that they are actually virgin mobile and they certainly won't offer you a contract on o2