GUYS PLEASE PLEASE READ THIS LISTEN UP IF YOU ARE STILL HAVING THIS ISSUE.... I got my iPhone 4S and it had a f***ed screen so I bought the replacement... Then all of a sudden, the phone signal stopped working... I initially looked to this forum and other forums on Google to find the answer, now some people are saying they have fixed it, that's great! But others haven't and here's why.....
IT'S BLACKLISTED/BARRED...
If no other steps fix your issue, do what I did and go to imei.info and purchase a simple blacklist check for a quid or so, and you'll have your answer... The guy I got the phone off a mere 5 months earlier, had actually did a lost insurance claim on it and the network blacklisted it.... So yeah guys, you really ought to check this first.... I couldn't believe I spent £ 120 for something that is as useless as a potato peeler in making 4G phone calls....
Sorry to all of you who are having this problem and come to the same conclusion as me.... :(