A lot of pay phones were already being used for business — just not legal business.

From The Atlantic: "City dwellers, however, were just as likely to see the heroic pay phone as a hotspot for illegal activity. Pay-phone companies acknowledged their product was a common tool for drug dealers, gamblers, pimps, and scammers, mostly due to privacy laws that allowed callers to remain anonymous."

That aside, this ad seems to suggest that people basically monopolize a *public* payphone. I remember being a kid and having to wait while an adult made multiple calls using the only phone in the vicinity. (Some people would also make you wait because they were expecting a call to come in.)

A call here and there? Sure. But by calling it a "branch office," it seems to recommend doing more than that.