As many others who have posted, when I received my replacement SIM and fitted it to my Nokia 5320, I got the 'SIM not valid' message. Discovered that there is no way to access help from Virgin (789 send you a text link back to the website help - which doesn't). After search on this forum and the web, discovered the unlock code instructions and tried this using the unlock code sent with my new SIM letter - got 'code error' message twice, not prepared to try again due to '3 strikes and your locked out' warnings on the internet.
Found link to virtual assistant on Virgin website - this is where it gets farcical...
Described issue to virtual assistant - got text telling me to put in the new SIM card and it would work... Replied that this didn't answer my question. Gave my name and number details - was told I would be transferred to one of the team - confirmed 'yes' that I wanted to keep the conversation open. Waited for the rest of the day - no contact.
Next morning, message from Virtual Assistant - just check if you still need some help - replied yes. There then followed a repeat of the previous day's texts... Waited patiently again for the rest of the day - no contact.
Today - I no longer have network access.
Since there appears to be no way to get any support, I will have to switch to EE and cancel my Virgin Direct Debit.
Very disappointed after 20+ years as a Virgin Mobile customer.