Is there a limit to the number of telephones I can have hooked up to the same line?
What is the REN limit?
Yes and no. There really is no "absolute limit" on the number of phones or devices you can use on a phone line, but there is a limit on the amount of power available in your phone line. If you have too many telephone devices (including modems, telephones, answering machines, faxes, etc.), none of your phones will ring. Your callers will think your phone is ringing (they won't get a busy signal), but you won't hear any ringing!
Calculate your system's REN limit!
Your phone or device has a label on it, required by the FCC, that states either a value called REN or ringer equivalence. This number is an indication of the relative power needed to cause the phone or device to activate (ring, receive a fax, etc.)
Older phones with a physical bell or buzzer have a REN of around 1, while telephones with electronic tones have a lower REN. Computer modems, fax machines, and answering machines also have a REN value, and you should take them into account when figuring your total REN. For example, my answering machine also has a REN of .4.
In most cases, the maximum REM available is around 5.0. You can call the phone company to get your system's REN limit. Once you have the REN number, add up the REN numbers of all devices on the line. If they are less than the REN limit, you should be OK, if you exceed the REN limit, your devices may not respond to the ring tone. You may have intermittent problems if you get too close to the REN limit, so allow a little breathing room in your calculations!!
There are a two REN classifications you might see on your devices label:
You can see that the FCC label shown has both .4A and .7B classifications. This is because the phone has two ringers with different values, one for incoming calls and one for an intercom system. Since both ringers can conceivably activate at the same time, this phone has a total REN of 1.1.
Do You Have A REN Deficiency?? Troubleshooting REN problems...
If you think your phones are not ringing because you have too many devices (and can't or don't want to call "THE MAN" at the phone company), troubleshooting is as simple as unplugging devices till your system accepts calls. Or unplug ALL devices and replug them into the system ONE AT A TIME until your system can no longer accept calls.
Back in the day when I wrote this article, you couldn't easily do this unless you had two phone lines OR a friendly neighbor. Now with the wide availability of cellular phones, troubleshooting REN problems is a snap!!