My new SIM card isn't working?

I recently got a SIM card from virgin media for my iPhone 4, they said they would cancel my old contract and activate my new SIM card, 48 hours later my new SIM says it is activated but says no service at the top of the screen. And my old SIM card that should be deactivated is still working!

Contact the center once… There might also be a problem with your sim card slot.