Hi
A few years ago we had our car stolen. One of the things in the car was my Blackberry. So I called T-Mobile (the network it was on) and they blocked the IMEI.
Within a couple of days the police found the car. However as the car was a company car and it was leased, the police had to return it to the lease company and then they had it taken to a garage to have the locks changed etc. So it was a couple of weeks before we got the car back or had access to it. When we did my phone was still in it.
So I called T-Mobile and they were able to un block the IMEI and I had my phone back. The contract ended and I wanted to sell the handset on to a recycling company but no one would buy it as the imei had been at one point blocked and reported as stolen.
If you were the customer that the handset was supplied to, then I think you might be able to get O2 to remove the IMEI block. However whats to stop someone stealing a handset, holding onto it for a month or so, until the owner gets a replacement from the insurance. Then calling the network and saying I found this phone and when they say they are not interested asking for the lock to be removed under the excuse that they want to sell it for charity. I know this is not wants happened or what you are trying to do, but it would be a way to get round the issue of the IMEI lock.
Best bet is to hand it in to the police or if you don't want to do that, stick it in an envelope and mail it to O2.
Alex
iPhone 4s black 16gb (O2), iPhone 4s white 16gb (Vodafone)