A Homemade Basic Key System with OBi's: Part 1 - Overview

Post date: Jun 7, 2016 5:30:01 AM

My current project is getting my in-laws' house wired for phone service. They retired to a remote lake house where Internet access can only be had via satellite and mobile service is spotty. The house is a three-story structure with a huge detached garage. The goal for the phone is to provide the ability for the house to call the garage, and vice-versa.

For this project I took the opportunity to upgrade my telephone system to an OBi202. With that in place, my old OBi100 was freed up to join the OBi110 I pulled out of my workshop to make a simple key system. There are 3 major features to this system:

1) Simple Private Branch Exchange - The essential feature for this system is for the two ATAs to call each other using only the LAN with out the help of the Internet. If you have a reliable Internet connection, dialing between OBi endpoints couldn't be simpler. Every OBi has a unique identifier and you just dial **9xxxxxxx and it rings. Removing the Internet from the equation makes the whole affair rather complex. Lastly, since I will be supporting this system from two states away, I don't want to run any servers. I just want to drop these boxes onto the network and basically be done with them.

With this system, the OBi110 will sit in the house and the OBi100 will go in the garage. The house will be able to call the garage, and vice versa using only the wired Ethernet that connects the two buildings with no assistance from the Internet or from a server on the network. This will involve programming each box to run its own SIP server, to allow connections from the other box, and setting up a special speed dial, '99#', for each box to call the other.

2) Shared Access to the PSTN - In a business setting, you call inside the company's private phone system by dialing a 3-digit extension number, or you dial '9' to get an outside line to call the public telephone network. The OBi110 comes with a phone port for connecting an analog telephone, and also a telco line port for connecting a landline or other phone system. This setup will share the telco line port between the two OBis allowing any phone connected to either OBi to make outgoing calls.

This has two benefits. First, it simplifies the setup of the outside line service. Adding a landline, or a VOIP service to the system is simply a matter of plugging it into the telco line port on the OBi110. For testing purposes, I have been using a Bluetooth adapter that allows a mobile phone to connect to an analog network. Whatever phone solution my in-laws settle on, they should be able to deploy it themselves.

The second benefit is simplified dialing. With most business PBXes, calls to outside lines require a prefix, which is most often '9'. With this setup, the only special number to dial is the other extension, which for both the house and the garage is '99#'. To call out of the system, you simply dial the number and wait a few seconds for the call to route. Other than the delay, calling is identical to using a landline.

3) Extensions accessible from the PSTN - When a call comes into a PBX, the system has to decide where the call goes. This is called routing. Calls are routed when they come in and go out. The last feature of the basic key system is to have a call from the public telephone network ring all the phones attached to either the OBi110 or the OBi100. This way, it doesn't matter if you are in the house or the garage, you can pick up an incoming call.

Some key systems require you to press a button, or dial a code to grab a ringing line. For this system, which really only has two users, none of that is necessary. When a call comes in, the phones in the house and the garage will ring, and whomever picks up the phone gets the call.

Now that the system is designed, the next step is to start gathering and preparing the pieces. I will cover that in my next post, Part 2 - Preparation.