Lucent Technologies PARTNER Advanced Communications System Programming And Use Instructions page 31

Requirements
An industry-standard device must meet the following conditions:
It must be nonproprietary. That is, it cannot be made specifically for use on
a particular telephone system. (For example, you cannot connect a Lucent
Technologies MERLIN LEGEND® Communication System phone,
because it is specifically designed for use on a MERLIN LEGEND
Communication System.)
Its Ringer Equivalence Number (REN
REN is shown on a label on the device, usually on the bottom.)
You can connect a standard two-line device to the system, but for best
results it should be installed and used as if it were a single-line device.
Connecting Standard Devices
You can connect a standard device 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 REN of the two devices together does
not exceed 2.0. (System phones have 0.0 REN.) For example, you can connect a
standard phone and an answering machine to the same extension. An extension
with two devices connected to it is called a combination extension . You cannot
connect two system phones on one extension. The PARTNER Advanced
Communications System Installation guide provides installation instructions.
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REN is a measure of the power it takes to ring a phone. The typical home phone line
supports 4.0–5.0 RENs; each extension jack in your system handles up to 2.0 RENs.
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Overview
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) cannot be greater than 2.0. (The