Blocked iPhone has signal again?

So I bought a iPhone4 from eBay with a cracked screen which I fixed but then found the phone was blocked by the network so after trying to get my money back for ages I gave up. I just kept it as a satnav and iPod touch as Wi-Fi and everything works and for some reason I can use 3G aswell. I called up 02 and they said it was blacklisted but wouldn't say why as I didn't have the original receipt. So I did a check mend search and it came up saying it hadn't been reported lost or stollen and it hadnt been subject to an insurance claim. So anyway I just use an 02 sim in there for 3G but I can't make calls or anything. But then suddenly today the signal came back and I'm able to make calls? This happened once before but then i turned it off and on and it went back to being barred. Can anyone explain this? I'm confused

Added (1). Oh really tanks but iPhones are very different i did try to get it unlocked but they make it very tricky. But the thing is it is working fine right now and I can't understand as I havnt done anyhting to it. Is it just because 02 system is down or something. How does blacklisting work?

Added (2). Thanks sonny that makes allot of sense, maybe whoever first bought it didn't pay there contract and e dept collectors have finally been round. It's anoying because I can't get hold of the original owner and ask them. I guess they broke Phone and sold it on eBay and didn't want to pay the contract anymore so it got blocked but wy got made to pay finally which means I have a working iPhone 4 for now.

Because the phone is blacklisted this is te main reason amd this also happened when i brought a cracked screen blackberry bold 9780 it won't work but it shall cost u 40-50 pound unlock the phone akd it will be working as a normal iphone again

This can be done at phone shops or at markets for a cheap proce

I've had my iPhone 4 blocked before because I'm on contract and couldn't afford to pay it, mine was completely uncontainable, what happens is once you pay the money and you speak to customer service to get your phone back on, you have to turn your phone off and back on to update the settings, that's why the signal came back, you have to do it to update all your settings, had this experience for months and it was a royal pain paying up, that's your answer, and I have a nice crack on my screen too, it can also take up to 24 hours to update