REN

REN = Ringer Equivalency Number

A rating for FXS ports that indicates how many phones can be simultaneously connected to the port.
In days past, the old mechanical ringer telephones had REN ratings for 1.0 each, and there was a limit of 5 phones per line.

Many modern day electronic phones have RENs of much less then 1.0, although there are a few that draw all their power from phone line, and have RENs of more than 1.0.

VOIP ATA FXS ports may support a total REN load of less than 5.0, so if connectiing multiple phons to a ATA >FXS port you should verify that the sum of the RENs of all phones connected is less then the REN load supported by the FXS port.

REN Boosters are available that raise the total REN load that can be supported on a FXS port. So if you need to connect a device or devices with a high total REN to a ATA FXS port, one of these devices may solve the problem.