The E-10A, E-10-WHA and E-20B are telephone line powered speaker phones that provide straightforward two-way handsfree communication. All units function identically, but are available in different housings to better suit the application. When the "Call" button is pressed, the speaker phone will come off-hook and will remain off-hook until one of the following happens-a CPC signal is detected, the programmed timeout has elapsed, a busy signal is detected, or the call goes silent, making the unit automatically disconnect.