Connecting Industry-Standard Devices
You can connect an industry-standard device such as a fax machine, modem, or answering machine so that it is on an extension by itself, or so that it shares an extension with another piece of equipment (either another standard device or a system phone) as long as the device meets the following conditions:

● It must be non-proprietary. That is, it cannot be
  made specifically for use on a particular telephone
  system and must be capable of being connected
  directly to the public telephone network 
● Its Ringer Equivalency Number (REN) cannot be
  greater than 2.0.
   ● REN is a measure of the power it takes to ring a phone 
   ● The REN is shown on a label on the device,
     usually on the bottom
● You can connect a standard 2-line device to the system,
  but for best results it should be installed and used as if
  it were a single-line device. For example, you can connect
  a system phone and an answering machine to the same extension.

Installing Out-of-the-Building Equipment
You may wish to install a telephone outside of the building. For example, a phone for a night watchman who occupies a booth near the entrance to the property. If any wiring from the extension jacks leaves the building premises, you must install In-Range-Out-of-Building (IROB) protectors to prevent damage from lightning.

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