m0ei wroteThe phone itself is the problem....
No it's not the phone. It's the area where you're in.
I have exactly the same Iphone4, iOS 5.0.1 jailbroken and factory unlocked and also subscribed to MTC 3.shit kiloGrams of crap and I face the same issue.
In Beirut, when I enable 3G I get my full reception bar but the internet is shockingly spontaneous. If you're lucky, web pages open fast otherwise it will take AGES to load.
I go up to my home in the Metn area and once I enable 3G I lose the signal and I can't make/receive calls let alone open a web page.
Long story short, keep Edge enabled and turn 3G off. Save yourself some time and don't bother trying.