Why is my iPhone getting No Service?

So I found an iPhone 4 and waited for the owners to pick it up however they never did, so I kept it. (So don't start with your TAKE IT TO THE POLICE crap)

The iPhone is in good condition and everything works perfectly well, however it is jailbroken. It's getting No Service. I've tried restoring it, getting Ultrasn0w, restoring with Sn0wbreeze, ect. I don't really know what to do now. I've not tried downgrading but I'm about to, does anything else work? Any help would be amazing!

Added (1). Also it was on an O2 sim before, I have tesco, which works with O2 phones.

Some network are rubbish or some areas can't u change networks
mines on 3 and spends a lot of its time on no service and my mate had tesco not great service
O2 has a really good service