This extension booster increases the REN capacity of a normal telephone line from 4 to 11, allowing you to fit many more extensions around your home. Every piece of telephone equipment which connects to the telephone network has a ringer equivalence number (REN). This defines the amount of current it draws from the telephone line. Most equipment unless otherwise stated has a REN of 1. To determine how many pieces of equipment can be added to a line simply add up the REN numbers of each item. The capacity of a normal BT line is REN 4.

Note that this equipment id not rated for use with broadband activated lines.