The E-10A and E-20B are telephone line powered entry phones designed to provide two-way handsfree communication. Both function identically, but are available in two different housings to better suit the application. When the "Call" button is pressed, the E-10A or E-20B will come off-hook and will remain off-hook until 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.