Read this as one signaling leg
The diagram shows a simplified direct exchange between two SIP participants. A real PBX often terminates one leg and originates another, so the phone-facing and carrier-facing exchanges can differ. Not every call contains every provisional response shown here.
INVITE requests a session. Provisional responses describe progress. A successful final response to INVITE establishes the accepted session parameters, and ACK confirms receipt of that final response. Audio can then flow on the negotiated media path; early media can occur before final answer in supported flows.
Media is a separate path
SIP messages commonly carry SDP to negotiate addresses, ports and media formats. RTP carries the voice and can follow a different route from signaling. A working INVITE exchange therefore cannot prove that the receiver heard anything.
When a call has one-way audio, compare the SDP on each leg with actual packet delivery. Do not infer the media destination from the address of the SIP proxy alone.
Ending and cancelling are different
BYE ends an established session and receives its own response. CANCEL attempts to stop a pending request before establishment. Race conditions matter: a call may be answered while cancellation is in flight, requiring the established dialog to be handled correctly.
Use the sequence diagnostically
Find the first point where observed behavior differs from expectation. No reply to INVITE suggests a different investigation from a final rejection or an answered call without media. Correlate request method, identifiers and direction. Keep the full bounded sequence instead of reporting only the last response number.
Watch the explanation
Learn the difference between turning Do Not Disturb off and removing the user controls that enable it. The demonstration examines the idle mute-key action and the separate DND feature setting, which can matter when users accidentally stop their phones ringing.
Read applicability and editorial notes →Watch this video here
GRP-family web interface; the spoken model number is unclear. Match the controls to your exact model and firmware before applying a policy change. Review used the available transcript and primary references; audio and screen readability remain unverified.
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Sources & applicability
Primary references for the technical details above. Operational examples and planning checklists are VoIP.info editorial guidance.