I've inserted a new replacement sim - an onscreen message asks for a "Network Key", however there is no mention of this key in the accompanying letter. I've sopoken to call centre staff - they said I would receive the key via email. Four days later and the email containing the key hasn't arrived. My old sim has been deactivated so there's no point in putting it back in.
It sounds very much that your phone is network locked. The original SIM cards were EE based and the phones were locked to EE type cards. The new cards are Virgin's own and are seen as a different network by the phone.
As this is one of Virgin's own phones they should be able to supply you with an unlocking code.
Bizarrely, I've spoken to a member of Virgin's 'customer service' staff this morning and she claims to have never heard of a "Network Key".
She argued that I'd read the text message incorrectly, and it actually said "sim code". Well the message definitely refers to a "Network Key" - NOT "sim code" but she continued to dispute this FACT.
I pointed out that I'd already noticed at least one other comment on this forum from a customer who has experienced exactly the same problem ... so perhaps it would be better if she transferred me to a more senior member of staff. She refused ... in fact she reckoned that she was more familiar with technical terms that her colleagues who hold posts higher up the employment scale!