Sylvania STC590 Owner's Manual page 37

FCC Requirements
RINGER EQUIVALENCE NUMBER
NOTICE: The Ringer Equivalence Number (REN) assigned to each terminal device
provides an indication of the maximum number of terminals allowed to be connected
to a telephone interface. The termination on an interface may consist of any
combination of devices subject only to the requirement that the sum of the ringer
equivalence numbers at all devices does not exceed 5.
CAUTION:
Users should not attempt to make such connections themselves, but should contact
the appropriate electric inspection authority, or electrician, as appropriate.
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