An immediate factor that distinguishes a ringdown phone from a standard phone, is the lack of a keypad (see inset). This is due to the fact ringdown phones are built for calling within a ‘ringdown circuit’ or end-to-end connection. Taking functionality in a different stride, ringdown circuits work in several directions, or can be setup so that phone A will only ring phone B, and not vice versa, along with various other options I’ll get into in a moment. In dangerous conditions or environments where this limited calling ability is all that is needed, these phones are typically preferred (control towers, mines, offshore drilling rigs, refineries, etc.). Ringdown phones are best considered as one-way hotlines, devoted to taking the time out of dialing back and forth, especially when speed or convenience would be desired between individuals for time-sensitive information or even emergency response.
Traditional ringdowns used to function by signaling an operator through a pulsed AC signal (either with or without a switchboard). In non-operator use, they send pulsed signals from end-to-end by simply picking up the phone to connect – no dialing needed. In VoIP ringdowns, as soon as the handsets are lifted, calls will be automatically routed to their assigned location via PBX programming routes. Whereas traditional ringdowns needed to be hard-wired, VoIP ringdowns take advantage of routing via web addresses as well as SIP protocols.
Another method of VoIP Ringdown calling involves Private Line Automatic Ringdown (PLAR), PBX Direct Line, (often associated with the Batphone [circa 1966]) – creating a permanent association between a single voice port and destination number. With PLAR configured, when a VoIP phone goes off-hook, a pre-defined call is initiated to another voice port or number immediately. Additionally, Ringdown phones operate off telephone line power and are heavy-duty, built for the conditions they often exist in, needing to meet Zone 1 safety requirements.