unblocking a lost iphone 4s after found??? help!!!

Hey guys I live in the uk and i have an iphone4s on 3 network. I was out a few days ago and lost my phone called my network to block the sim card and contacted my insurance company to get a new phone. turns out the new phone they've given me is a refurbished/repaired one and i'm having loads of problems with it. a few days later my mother received a phone call and an old women has found my phone so i go and fetch it and as i said it's blocked. what can i do to get it unblocked because this phone is working fine and the replacement one doesn't the insurance company is terrible and want my phone back and won't have the refurbished one back. HELP PLEASE!!!