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Collaboration, Voice & AV · Foundational

Foreign Exchange Office (FXO)

An analog interface that connects to a PSTN line or PBX station port and receives dial tone rather than providing it.

Definition

An FXO port behaves like a telephone: it goes off-hook, receives dial tone, detects ringing and sends digits toward the central office or an FXS port. Voice gateways use FXO ports to connect to analog CO lines in small sites or for SRST fallback trunks. Because analog loop-start lines provide weak disconnect supervision, FXO ports may require disconnect tone or battery-reversal configuration to avoid hung calls.

Example

A branch router uses two FXO ports to analog CO lines so that local 911 calls work during a WAN outage.

Exam tip

Watch for scenarios about calls not disconnecting on FXO trunks; the fix involves supervisory disconnect or answer/disconnect supervision.

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