iphone3g no signal but still has edge &3g working ??

[galapunk]

hi folks,
i have a iphone3g locked to orange, had it for a year or so, it has been barred for some reason (prob bumpt) when i first got it it was still on firm 3.1.3 (never updated) so i did the blackra1n jailbreak and unlocked it (thats how i knew it was barred, no signal from any networks) but i found for some reason if i put a orange sim in another phone and let it find the knetwork, then put the sim into the iphone i get a full signal, can call and recieve calls, use edge and 3g,
But when it looses its signal or i turn it off and back on it has no signal at all, cant call out or recieve calls or texts, BUT i still have edge and 3g and am able to use the web and any other app needing a network connection ????

has anybody else came across this ???
it is now on 4.0.1 and it still allows me to do the same ???

its had me baffled for a year, i hope someone can shed some light on my problem..

thanks in advance...
meiny aka...nastynasty