Idle state:

Tip = open, Ring = CO Battery

Outgoing Call:

PBX Off-Hook Seize message: Places ground on Ring

CO response: Places ground on Tip

PBX response: Close the loop

CO response: Provide loop current

PBX response: Dial out digits

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

CO response: Open circuit on Tip

CO On-Hook first Disconnect: Open circuit on Tip, no loop current

PBX response: Open Tip-Ring loop

Incoming Call:

CO Off-Hook Seizure: CO applies ground on Tip and applies ringing voltage to the Ring lead.

PBX response: Make trunk busy for outgoing calls

CO Ringing: CO applies ringing voltage to the Ring lead.

PBX response: Detect ringing, ring destination (Attendant, hunt group, etc.)

PBX Off-Hook Answer message: Close loop

CO response: Trip ringing, provide loop current

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

CO response: Open circuit on Tip

CO On-Hook first Disconnect: Open circuit on Tip, no loop current