Thanks for your post on our Community Forums and a very warm welcome to you!
Sorry to see you've been struggling with the new SIM card, is this issue still on going or has this now been resolved?
It sounds very much like you will need to have the handset unlocked, as it may be locked to another network - this could even be EE if the handset was purchased from us. The Red SIM cards were EE hosted, as oppose to the new Purple SIM cards that are controlled by Virgin Media.
Was this discussed with you over the phone? If so, were you issued with the unlock code?
Can you also please advise me where the handset was purchased?
It would also be handy to know if the handset is a touchscreen or if it has physical buttons, please advise me further if you require assistance