In terms familiar to people knowledgeable about phone systems, the PBX was connected to a digital telephone service known as a T1, configured as Primary Rate Interface ISDN (integrated services digital network) or PRI ISDN. What this meant was that every time a call was placed from Panda, setup and other call processing information went out over a data channel to the phone company's switch; the information included the calling party number, which (unless blocked) is delivered to the caller ID device at the receiving end.