I wrote the article, I think that can be cleaned up. The Pi Served as the payment relay, which would forward to the payment processor. The RING RING is actually generated by the auto-dial on the modems, which were set to auto answer every time the ATMs dialed. Its a visualization of what the modems are receiving from the ATMs

In a "real world" example, the Pi would be the Localized Payment Processor which then forwards transactions to a remote one.