Private line automatic ringdown
One of the more practical features of telephones is that they behave like they're multiplexing. In other words, the vast majority of users need only a single phone, which they use to dial any other phone. You don't need a separate physical phone for each remote phone you wish to dial. However, there are many situations where it makes sense to have a dedicated phone, which can only dial one other phone.
These situations boil down to two categories: security and speed/convenience. For example, in one situation, you might have a phone in the outer door of a secured facility. You may want visitors to be able to pick up this phone and be automatically connected to the security department or the front desk, without having to dial, so that they can be escorted into the facility. This is especially useful in a campus situation, where the security might be centralized, but responsible for dozens of buildings. In any case, you would not want anyone to be able to walk up to these phones and start making personal calls to the Psychic Hotline.
An example of a situation where speed or convenience would be desired might be communication between people that work on very time-sensitive issues, like commodities or securities traders. Two traders that work together extensively might want a dedicated line, so that when one picks up the phone, it automatically rings the other's phone without the need to dial.
This feature is commonly known as Private-line Automatic Ringdown, or PLAR. It's implemented in many regular PBXs, but also in most VoIP equipment, where it can be incredibly useful. Fortunately, the configuration is quite simple as well. For example, on a Cisco router equipped with voice ports, the command to configure one phone to dial another automatically is just "connection plar 5551212" where 5551212 is whatever phone number or extension you want to dial automatically.
Also note that this feature is really one-way. For two-way connectivity, you'd want to put a "connection plar xxxxxxx" command on the other phone, pointing to the first phone's number. This is important because one-way PLAR is often useful. Consider a situation like users on a factory floor or in a retail store. You might want to configure several phones out on your floor so that they automatically dial the on-duty manager, but you wouldn't want the duty-manager's phone to automatically ring one of those phones, for obvious reasons.
Because a lot of these types of communications are intra-company, they're particularly well suited for VoIP applications, because there is often an existing LAN or WAN connection between the endpoints.
Thomas Alexander Lancaster IV is a consultant and author with over ten years experience in the networking industry, focused on Internet infrastructure.