Can I unlock a blocked imei on an iPhone 4?

Hi everyone, i need some help. My sister found an iphone 4 at her work a few months back. She handed it in to lost property. Her work phoned the home number on the mobile, left messages to say that they had found the phone but the owner never bothered doing anything about it. After two months my sisters work as per policy gave the phone to her to keep as she handed it in. So this is not a stolen phone, just want to make that clear. Now i've managed to wipe and reset it but the IMEI has been blocked, changing the IMEI is illegal, what are my options for getting this phone to work properly again?

HI, I know your not going to want to hear this but there is no way round this unless you take it to another country. Usually a network can only imei block a handset in their country.
There is no way to remove this bloack other than for the original owner to contact their network with their account details and ask for the block that they put on the handset to be removed. Thats the only way. Sorry.

It is not possible.