Refer to the exhibit. A user reports that when they call a specific phone number, no one answers the call, but when they call from a mobile phone, the call is answered. The engineer troubleshooting th

Question

Options

• AAllow Passthrough of Configured Line Device Caller Information must be enabled.
• BAccept Audio Codec Preferences in Received Offer must be set to On.
• COn the SIP Profile, the configuration parameter SIP Rel1XX Options must be set to Send PRACK for all 1xx Messages.
• DEarly Offer for G Clear Calls must be enabled.

• A
  11% (4)
• B
  6% (2)
• C
  80% (28)
• D
  3% (1)

Explanation

A SIP 183 Session Progress message is a provisional response (1xx) that often carries an SDP body for early media (e.g., ringback tone or IVR audio from the PSTN). RFC 3262 defines PRACK (Provisional ACKnowledgement) as the mechanism to reliably acknowledge provisional responses. If the far-end gateway sends the 183 with SDP and requires a PRACK before cutting through audio, but CUCM is not configured to send PRACK, the audio path will never be established. Setting the SIP Profile parameter 'SIP Rel1XX Options' to 'Send PRACK for all 1xx Messages' instructs CUCM to acknowledge all provisional responses including the 183, which allows the far-end gateway to proceed with cutting through audio on the early media path.

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