I don't know why this gets attributed to MGS3 so much, it doesn't work like MGS2. The MGS3 alert is almost instant, with all guards in the area made aware. The call just brings in reinforcements.
I'm not sure if I'm remembering it right, but in MGS2 guards run away to call in the alarm as soon as they spot you, while in MGS3 they would first yell something like "Intruder!" and then move to cover while shooting to make the call, no? If I'm right the yelling and shooting would obviously be what alarms other guards.
I might just be completely wrong on this, though, to be honest.