If the customer has dropped the contract after 3 months the handset could be blacklisted as most mobile companies do this and is unable to be unlocked. Are you trying and the handset seems to be working normally and when you try to make a call and then you get beeps and the call is dropped?
Try here and check this out as NOKIA as they have the best system to stop them been unlocked again after blacklisting.
http://www.unlockme.co.uk/blacklist.html