7210 weird 'sim registration failed' issue

[Viking]

You will have to excuse me, I am a real Newb to this so if I have posted in the wrong section or if I have any details wrong, please feel free to correct me.

I am attempting to use my O2 SIM card in an unblocked Orange 7210 phone.
The phone was unblocked using a software unblocking tool ( DCT4 I think )
This worked absolutely fine for a few days but now when I enter the O2 SIM card I get the message "SIM card registration failed".
I understand that this can be due to the phone being black listed but I know exactly the phones history and there is nothing 'dodgy' at all. Also I know the phone hasn't been black listedit because if I put an Orange SIM card into the phone it works.
Can phones that have been Software Unblocked revert back to have the blocks installed ?

Any help you could offer would be appreciated.