VoIP.info Guide

FXS vs FXO: connect the right analog roles

Understand line-supplying and line-consuming ports before choosing an ATA or analog gateway.

Reviewed 2026-10-27Analog interface and carrier behavior vary by device, region and firmwareIntermediateDocumentation based · not lab tested

The roles are complementary

An FXS interface supplies the analog telephone-side service, including line feed and ringing behavior. An FXO interface behaves as the line-consuming side, as a conventional phone does toward a telephone line. To connect an analog phone to a SIP service, you normally need an ATA's FXS port. To bring an existing analog line into an IP PBX, you normally need an FXO gateway port. Devices such as the HT813 provide both roles in one unit, but the ports are not interchangeable.

Multi-line gateways such as the HT841 and HT881 combine several FXO ports with one FXS port. Count and label the two roles separately.

An ATA FXS port supplies an analog phone interface. A carrier analog line supplies an FXO gateway port. Both gateways connect to SIP, but their analog roles are opposite. — FXS supplies the line service; FXO uses it. These are two separate connection examples. Never connect two live FXS sources together.

Check more than port count

For FXS, verify ringing load, supported caller-ID format, country impedance settings and the attached device's dialing behavior. Multiple physical ports do not necessarily share one SIP identity or provide the same simultaneous-call capacity. For FXO, verify line supervision: how answer and disconnect are detected, whether polarity reversal or current interruption is supplied, and which tones and cadence apply. A line that remains seized after hangup may have a supervision mismatch rather than a SIP failure.

Avoid damaging assumptions

Do not connect two line-supplying interfaces together or feed an ATA into building wiring that is still connected to a live outside line. Have the wiring isolated and identified by a competent installer. For selection, record the required port role, regional electrical profile and call behavior. Then test ring, answer, DTMF and disconnect in both directions. A dial tone alone is not enough: it may be generated locally even when upstream service is unavailable.

Identify every GXW4200 v2 socket as FXS

The GXW4200 v2 family guide explains why these subscriber ports must not accept an outside analog line.

Confirm the HT81x V2 ports are FXS

The HT812 V2 and HT814 V2 supply FXS ports for analog endpoints; they are not FXO interfaces for connecting telephone-company lines.

Use current HT841 and HT881 port-role examples

The HT841 and HT881 show why PSTN-facing FXO ports and device-facing FXS ports cannot be interchanged.

Use HT813 as a one-to-one role example

On HT813, FXS powers the phone or fax while FXO faces the PSTN or traditional PBX line; swapping them changes the electrical and call boundary.

Map ports, spans and channels explicitly

The AMI DAHDI guide keeps FXS or FXO port identity distinct from DAHDI span, channel, group and call leg.

Sources & applicability

Primary references for the technical details above. Operational examples and planning checklists are VoIP.info editorial guidance.

Examples require adaptation to your topology. No live PBX or hardware testing is claimed.

The roles are complementary

Check more than port count

Avoid damaging assumptions

Identify every GXW4200 v2 socket as FXS

Confirm the HT81x V2 ports are FXS

Use current HT841 and HT881 port-role examples

Use HT813 as a one-to-one role example

Map ports, spans and channels explicitly

Sources & applicability

Continue your research

Connecting an analog phone to VoIP with an ATA

Connecting an analog line to a PBX through FXO

Grandstream HT813: one FXS phone, one FXO PSTN line and lifeline routing

Grandstream HT841: four FXO lines, one FXS port and lifeline mapping

Grandstream HT881: eight FXO lines, one FXS port and lifeline mapping

Grandstream GXW4200 v2 FXS gateways: ports, profiles and firmware boundaries

Grandstream GXW4200 v2 port registers but rings wrong port or cannot call

Grandstream HT812 v2: two FXS ports, profiles and firmware boundaries

Grandstream HT814 v2: four FXS ports, profiles and firmware boundaries

Grandstream HT812 V2 or HT814 V2 port registers but phone, fax or audio fails

Grandstream HT841 or HT881 routes the wrong PSTN line, loses lifeline or stops calls after firmware

Grandstream HT813 FXS, FXO, lifeline or PSTN call route fails

Asterisk AMI DAHDI span and channel inventory with alarm evidence

Asterisk AMI DAHDI span or channel is missing, alarmed or inconsistent