Is my nokia 6300 locked?

Hi there

I bought an unlocked nokia 6300 in the uk which was initially locked with T-mobile. When I bought this mobile I tested it with a virgin sim card and a lebara sim card, and both worked fine.

I'm now in africa and can't get the phone to work on a sim card I bought here. The mobile is asking for a security code. When I put my virgin simcard back the phone work just fine.

Is this mobile locked or is it something else?

Thanks for your help