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SIP OPTIONS: reachability and capability checks

Interpret OPTIONS responses without mistaking signaling reachability for a working telephone service.

Reviewed 2026-10-09SIP standards; extension support depends on both peersIntermediateDocumentation based · not lab tested

What OPTIONS establishes

OPTIONS asks a SIP peer about capabilities without establishing a call. PBXs also use it for qualification: send a request periodically and measure whether a response returns. A successful exchange proves something about that signaling path at that moment. It does not prove the destination can receive a call, negotiate an acceptable codec or send audio through a firewall. A registrar may answer while a downstream endpoint is offline. Capture the responding address and identify whether the target is a phone, proxy, trunk edge or PBX before interpreting a green status indicator.

Reading a qualification failure

Compare the request URI, destination IP, transport and outbound interface with a successful call attempt. A request sent to an obsolete DNS address can make a healthy service appear unreachable. Conversely, an authenticated call may work when an unauthenticated OPTIONS request is rejected by policy. - No response: investigate routing, firewall state, transport establishment and the remote policy. - An error response: signaling returned; interpret the code and which device generated it. - Intermittent latency: correlate with packet loss and host load before shortening the timeout.

A useful operational check

Record the qualification interval and timeout alongside the observed round-trip time. Compare several samples during normal traffic and during a reported incident. Avoid aggressive intervals across thousands of endpoints without sizing the resulting request load. After correcting qualification, make an authorized test call with two-way audio, DTMF and hangup. Treat those as separate acceptance checks. The distinction prevents a successful OPTIONS reply from hiding an RTP or dialplan failure.

Keep ACS SBC-originated OPTIONS separate from call acceptance

The ACS Direct Routing diagnostic follows the SBC-originated OPTIONS request, Azure response and portal health indicator. Online health remains narrower than voice-route match, carrier acceptance, application event delivery or media.

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.

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