optoisolators 38 39 and 40 are interconnected to detect voltage and current levels input to the remote over the telephone line the current through resistor 36 indicates whether the telephone 13 is being utilized by a customer or otherwise has a closed circuit with the telephone line a condition commonly referred to as 34 off hook 34 current across resistor 36 saturates the transistor in optoisolator 38 which brings a hook switch signal low over hook switch line hooksw connected to a microcontroller 41 shown in fig4 the current across resistor 37 indicates either that a coin collect signal positive or a coin return signal negative has been transmitted from the central office over the telephone line to the coin phone 13 depending upon the usage of the phone 13 the direction of the current differentiates the above two signals and brings a collect or return signal low on an either optoisolator 39 or 40 respectively optoisolators 39 and 40 are shown connected to microcontroller lines return and collect and to receiver transmitter lines s ring and s tip 42 a solid state relay 42 is shown interconnected in the circuit the relay 42 provides the initial rate local call cost in order to dial a local call from the coin phone 13 at the remote location the relay 42 also has a line control function to open and close the outside telephone line the relay 42 interrupts the telephone line in situations when a customer takes the handset 21 off hook to make a call during a report phase between the remote 11 and host 12 thus the relay 42 restores a dial tone to the telephone line enabling the customer to make a call from the coin phone 13 the power supply circuitry.