This extension REN booster from BT, allows you to boost from a REN 4 to a REN 11. Enables additional telephone devices to be used. Mains power needed, connects to a standard BT socket. Telephone lines usually have a REN of 4 they can operate up to four telephone products e.g., phones, fax, answering machine each with a REN of 1. This product has a REN of 1. Can be used with Cordless phones, fax machines, computer modems. It is also compatible with BT ADSL Broadband lines. It is not compatible with BT ISDN lines Professional instalation may be required see PDF for installation instructions