I-Phone 4 Problems! HELP!

Sorry about the over dramatic title.

Received an iphone4 the other day, it's been "restored" via "dfu" mode and it's asking to do set-up, so I go through the steps, until it gets to "It may take a few minutes to activate your phone" then it doesnt activate...when I dont have a sim card in there and when I put one in it advises me that I need to put in one form the carrier network..

However, the person I got the phone from doesnt know the carrier (***hole right?) and I'm not stuck with a phone and I dont know what the carrier network is.

Is there any way to find out? I have used Itools and this doesnt tell me, also tried using a website sndeep.info/en which asks me for a Model number MC..... which I cant find anywhere on the phone, only information I can get off the phone is the IMEI and the Serial number. (Phone and network is UK)

Is it possible to get this phone unlocked for any/all networks? I've been googling last few days and this site repeatedly comes up, so I thought I might aswell sign up and ask the question.

Also,..what's the deal with this jailbreaking stuff? Does it really work? Reading alot of mixed reviews.

Thanks in advance,

Felix.