That means that he can only use one Bro Call on a bro, and his bro must respond to it UNLESS they have something that is equally or even more important than their bro's Bro Call.
If a person has already requested a Bro Call from a bro, he can ask for one more Bro Call from the same person. If that same person accepts the request, he gets an extra Bro Call. This can repeat as many times as that person accepts.
#2; "Is that a Bro Call?"
#1; "Yeah.. I really need to get a birthday present for my girlfriend but I have no ride."
#2; "Alright. I'll take ya."
The bro call ensures that guys can openly discuss a female's appearance without her knowing.
Hey man, check out that bro call, straight ahead.