This converts an RJ11 line with two phone pairs on it to two single line RJ11 jacks. It provides two jacks for each single line RJ11 line.

Typically the telephone cable in a house has four wires: Green, Red, Black, and Yellow. Green and Red can be used for one telephone line. Black and Yellow can be used for a second telephone line.

If you have two telephone lines you can get 2-line telephones that select one or the other. But while you can probably put one telephone line on hold (hopefully without dropping the call) while using the other, you cannot use both at the same time. The 2-line telephones are more expensive especially if you already have single-line telephones. Like I do.

That was my situation when I went to VOIP for my main telephone but kept my landline (downgraded to just local service). I did this because my ISP is not especially reliable even at the best of times. It also goes down during power outages. And while AT&T finally gave my community cell phone service I do not yet know how well it will work during power outages.

This phone adapter can be used in several ways:

1. Combining a landline with a VOIP line.

2. Combining two VOIP lines.

3. Combining a VOIP line with a Cell Phone using a adapter that uses Bluetooth to make a Cell Phone work with a standard corded or cordless phone. An example is the Cell2Jack. (Apparently this supports only a single ringer-equivalent phone.)

4. Combining a landline with a Cell Phone (with the above adapter).

Parts List

(5) RJ11 6P4C 4-pin PCB Jack

(4) M3x12mm (or so) Nylon Standoffs                    

(4) M3 Screws                       

When connecting my phone adapter it is essential that you use a cable with 4 wires on the 2-line side. (All the of the short cables that I had came from old modems and only had 2 wires in them.

The accompanying picture shows the connections in an RJ11 connector. Technically, when you use 4 wires it is called an RJ14 connector. This is from https://en.wikipedia.org/wiki/Registered_jack .

Note that the 2 wires in a telephone pair are called Tip and Ring. This goes back to when phone plugs were used in telephone switchboards. You (the subscriber) lifted up your telephone handset (and maybe turned a crank to produce the ringing voltage) and the operator would come on the line and say something like “Hello” or “Central” and then “What Number?” or maybe “Number Please.” You told her the number and she would pull a cable up from the console and plug it into the proper jack in the array of jacks on her console and connect you.  https://en.wikipedia.org/wiki/Switchboard_operator

These phone cables and connectors were also used in radio stations to route the audio to where it had to go (and could be easily changed).

These phone plugs are sometimes still used for headphones but the reason they are called phone plugs is because they were used in telephone switchboards.

 If you are not already a Maker (and you want to be one) see my tutorial on Making Things at www.jmargolin.com/making/jm_making.htm

JM