Annoying calls from VM?

I keep getting repeated calls from 08000 527551.

I'm advised each time I answer that the person calling is from Virgin Media. 

So far I've had 8 calls from this number, and each time I tell the caller that my contract does not expire until the end of the year, that I am perfectly happy with my present services, and will re-negotiate when the time is ready.

Can somebody tell me please if this is the sales team, and how does one make them stop?