By walking away! Give the two a loud and clear warning, hand gestures, no smile, hand on weapon, eyes on both of them..
Walk to the car, or walk to the nearest business, call 911 on the payphone!!! other options exists.. i believe what he did was right, but maybe he didnt exhuast all his resources. like maybe telling the guys he would call the cops.. I know this may be something us gun owners hate saying but it's an option, if that fails and they continue to move closer then act, maybe hold your hand out in the HALT position.. yell, never making a counter threat because then it becomes a fray.. basically two parties arguing.
If they move in for the attack, or draw a weapon, gun/knife/blunt object.. whatever, then draw!
And about the phone pimp thing...no ive never seen a phone pimp, but if these two think they own the phone, and want to charge for it's use...then so be it, let the police tell them otherwise... thousands of other payphones
----
Let's say you want to sit on a park bench, and a homeless guy thinks he owns it because he sleeps on it, and he tries to charge you to sit on it... would you stuff your snubby in his nose? or just go to a different bench!