Curtis TC924 - Manual (Page 34)

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 af 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.
FCC Requirements
33