7210 weird 'sim registration failed' issue

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 :confused: )
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. :confused:
Can phones that have been Software Unblocked revert back to have the blocks installed ?

Any help you could offer would be appreciated.

Sorry, just to clarify I meant I 'Unlocked' the phone just to remove the tie to the Orange network, NOT 'Unblocked'.

Hello :)
If you insert the SIM in a "locked" phone, it would say "SIM card not accepted".
"SIM card registration failed" means that the phone accepts the SIM but the network doesn't for some reason. Try your SIM in another (unlocked!) phone first if it works there.
I've had that with a pre-paid SIM which i didn't reactivate. After a month i could receive calls still, but could not call out even there was still credit on it. I still didn't reactivate. After three months he SIM still worked, but could no more get network (the number has been disconnected by the provider) and finally, after a year (!) the phone showed "SIM card registration failed", so i guess by then the SIM's data has been erased from the provider's database and the network could not identify the SIM any more.
As i said, try the SIM in another phone.....
Kind regards...
Thanh

PS phones that have been unlocked do NOT re-lock themself :)