There are other vendors, perhaps another vendor would be more interested. Was the shortfall so much that the company wouldn't want to make it up?
Perhaps the payphone could be put in the lobby or someplace accessible to more people than just the break room so as to do more volume.
Even old PBXs had a feature to limit outside calls to local numbers, not long distance; I would presume this kind of thing is still available. I would strongly consider a phone of this nature. Might be a lot simpler than putzing around with a pay phone.
One way payphones make money is through very high long distance charges. If one of the employees makes a collect or calling card call and subsequently discovers a $25 charge for a 1 minute call, they won't be very happy. The price of long distance is up to the property owner and need not be so high but then the phone might not pay for itself.
(The Philadelphia transit carrier, SEPTA, has Verizon pay phones at its stations with relatively reasonably priced coin long distance. NJ Transit, in contrast, has the high rates and no coin long distance.)