How to tell if a phone is Stolen - Network blocked : eBay Guides

 If the phone is on O2, Tesco or Orange it will power up ok but it wont get any signal & can’t make calls.

 If its on Virgin, Vodafone or T-mobile it will get a normal signal  & look as if it working ok but when you try to make a call it will reject the call.

Just to be sure its best to check with different sims just to make sure its not the sim that's faulty, also make sure that there is good reception where you are testing the phone  

You can get IMEI number of the phone checked at your local phone shop & they can tell you if its imei number is blacklisted (type *#06# to find the phone's IMEI number)

Not to be confused with phones being locked to a certain network and only working with that networks sim.  

if you phone is stolen or lost you can call your network provider and get them to "ban" the phone using its IMEI number.

 This renders the phone useless in the UK, regardless of whether the phone's sim is changed.