Reference / Voice
FXO interfaces for a PBX: an integration checklist
An FXO interface receives an analogue telephone line; an FXS interface supplies the line to a telephone or fax machine. Choosing the correct electrical role comes before choosing a PBX driver, expansion card or gateway.
Distinguish FXO and FXS
The FXS side provides line power, dial tone and ringing. The FXO side receives the line and signals when it goes on or off hook. An analogue telephone is on the receiving side; a PBX port accepting an outside analogue line also needs the receiving role.
Draw the path from telephone through PBX to the external circuit and label each port from the exact equipment documentation. Record regional line characteristics, connector type, caller-ID method, dialling and disconnect behaviour. Do not infer an electrical interface solely from a driver configuration label.
Commission in stages
Use a test line or approved maintenance window. Verify ringing, outbound dialling, two-way audio, disconnect recognition and recovery. Test voicemail or routing only after basic line behaviour is stable.
For IP media, verify DTMF deliberately. Telephone events may need a defined method instead of relying on a voice codec to reproduce tones. Claim compatibility only after testing the exact hardware, driver and service together.
Protect the live circuit
Telephone circuits can carry operational or emergency traffic. Identify the owner, maintenance window and rollback point before moving a line. Never treat an unplugged line as a harmless lab input.
Document how a user reaches assistance if the PBX, gateway or power fails. This is an operational requirement, not a feature claim about a particular card.