The iPhone consists of "two parts", a computer half and a modem half. Each half has an operating system. There is an OS for the computer half of your iPhone, and there's an OS for the modem half called the Baseband. The Baseband handles all things "cell phone" related. In order to unlock the iPhone to use it with non-official carriers (i.e. T-Mobile in USA) the Baseband is what must be hacked. An iPod Touch lacks the "Baseband" half, hence its inability to execute phone calls.