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