Idle State:

Tip = ground, Ring = CO Battery

Outgoing Call:

PBX Off-Hook Seize message: Closes the Tip-Ring Loop

CO response: DC loop current + Dial tone

PBX On-Hook Drop message: Open Tip-Ring loop, no loop current

CO response: CO goes to idle state (see Note)

Incoming Call:

CO Applies Ringing Voltage

PBX response: Detect ringing current

PBX Off-Hook Answer message: Close loop

CO response: Trip ringing, provide loop current

PBX On-Hook Drop message: Open Tip-Ring loop, no loop current

CO response: CO goes to idle state