Its very rare for a network to block the imei UNLESS the phone has been nicked. Im afraid if it has been nicked they wont unblock it for you. They could expect you to return the handset.
Is the phone branded ie vodafone o2 etc?? if yes and call the network up, They *MIGHT* tell you why its been imei blocked. The only network i know that blocks imei's for non payment is o2 and thats the iphone only. The rest dont unless its been nicked.
Quick question though what is the imei???
*#06# to display it.
If its 0000000000000000 then it hasnt been blocked its a bad flash, unlock thats screwed up the software.
What does the phone say when u put a sim in it??? Try o2 voda etc