I wonder if your original SIM was one of the old type. They were basically rebadged EE and have been withdrawn. The latest are Virgins own.
I suspect the phone is locked to the old type SIM.
Question is, who was the supplier of the phone and who is responsible for unlocking. I can see this bouncing around the two.
I'm a Very Insightful Person, I'm here to share knowledge, I don't work for Virgin Media. Learn more
Have I helped? Click Mark as Helpful Answer or use Kudos to say thanks