This circuit prevents you from getting your modem connections messed up by someone picking up a phone connected to the same telephone line of the modem.This circuit can be mounted in a regular telephone box. Note that it has an LED that will light when the telephones are disconnected. This indicator is very important because it will remind you to reconnect the telephones back when you are no longer using the modem. This happened to me once: I left home without switching the telephones ON, so I left my house phoneless, and THAT's exactly why I decided to add this indicator.

Remember that this box should be connected before the first phone in the house/office.