Iphone SIM card not supported?

So my girlfriend gave me her old iphone as my andrioid wasn't working correctly.

She is with a diffrent provider (unknown) and I'm with Virgin.

I placed my Virgin SIM card into the iPhone and it worked fine and has done so for months.

Recently Virgin sent out new SIM cards to all their customers saying we had to change.

At this point my old SIM card stopped working. I placed my new SIM card into the iPhone and it did not work. It said the following.

"The SIM card you currently have installed in this iPhone is from a network provider that is not supported under the activation policy currently assigned by the activation server. This is not a hardware issue with the iPhone. Please insert another SIM card from a supported network provider or request this iPhone be unlocked by your network provider. Please contact Apple for more information."

At this point it's good ot note that the new SIM card is the same provider and mobile number as what was working previously.

I contacted Virgin and they said all I had to do was backup my phone, reset the factory settings and place the new one in. This hasn't worked.

I'm unsure of my girlfriends provider but am fairly sure she has changed.

I'm unsure what to do and I need my phone fairly quickly, I read online on the Apple page that even if my girlfriend does request for the phone to be unlocked it could take a few days?

Any recommendations on what to do?

Added (1). Sorry I should also say, it's the exact same 'plan'/'contract' as what was working before.

How come?

Take it in to Virgin directly and have them see if they can get a sim card that works, or ask your Girlfriend who the provider is and figure out if it is supported by Virgin.