Activate iphone after changing sim card

Hi can someone please help me?

I have just switched from TMobile to Virgin. I have kept my old number with my new sim and so waited for my signal to go on my tmobile sim which it did this morning. Put my new sim in and it says 'Activation Required'.

It connects to the wifi but then says it cannot be activated because the server cannot be reached and to try connecting to itunes. but i am at work and cannot connet to itunes.

I have tried putting my old sim back in and the same thing happens.

Any ideas welcome.

Thank you