Telephone signaling interface

There are many interface standards between analog telephone central office equipment and customer-premises equipment. Single voice paths generally include analog audio connections, either a two-wire circuit or four-wire circuit plus signaling paths to indicate call progress and status information, such as ringing, answer supervision, etc. Some of the known interface types are

• FXO foreign exchange office
• FXS foreign exchange station
• DPO dial pulse originate
• DPT dial pulse terminate
• E and M signaling
• KS Kewlstart
• Loop start
• Ground start