Jailbroken iPhone - Different sim cards won't work?

I found an iphone 3G about a year and a half ago and tried to return it to its owner without any luck. Anyway I jailbroke it the other day with the latest version of redsnow using 4.2.1 and everything works find except for making calls and texts. It just either says sim locked or searching at the top left hand side of the screen, depending on the sim I put in.

A phone store said the firmware I'm using is too recent. Could this be the case?

Any help please and thank you

Why did you download jailbreak

Jailbreaking doesn't unlock iphones.
4.2.1 is the highest firmware a 3g can use.

Yeah, jailbreaking doesn't unlock an iPhone. Simple.
You'll want to go into cydia and download ultrasn0w to do that, the only thing, ultrasn0w might not work the the latest iOS version. However, if you "found" this iPhone, the chances are incredibly high that the iPhone's id number has been blacklisted as stolen, meaning it will be useless regarding of whether its unlocked or not.

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