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Overview

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supports Call Back When Free (CBWF) feature, also known as Ring Back When Free (RBWF), for UK-ISUP, IUP, SIP-I and SIP interworkings. When SBC is configured for CBWF support (callBackWhenFreeSupport flag), a call is automatically established to a destination number that was previously busy without the need for the calling party to redial the number.

set profiles signaling signalingProfile <profile name> egress egressFlags callBackWhenFreeSupport <disable | enable>

 

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, Sonus GSX and ASX support CBWF for the following protocol interworkings (see call flow diagram):

  1. SIP-I (UK-ISUP) to BT-IUP interworking over GW with Sonus GSX.
  2. SIP to SIP-I (UK-ISUP) interworking
  3. SIP-I (UK-ISUP) to SIP-I(UK-ISUP) interworking
  4. SIP to BT-IUP (supported on GSX only)

 

SIP Protocol Interworking Using CBWF

 

CBWF is implemented on SBC using four separate but related signaling sequences described in the table below.

CBWF Signaling Sequences

Signaling SequencesUsage
Call Back When Free - RequestCalling party requests the registration of CBWF at the Originating and Terminating PBXs.

Call Back When Free - Free Notification

Notifies the originating network about the status of the called party.

Call Back When Free - Call Set-UpEstablishes call between the requesting and desired extensions. This follows the receipt of a Free Notification when the extension is/becomes free.

Call Back When Free - Cancel Request

The calling party cancels the call, or when a CBWF Request is canceled as the result of a timeout.

Additional CBWF Configurations

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also supports following configurations:

  • Sending CBWF-related APP (ACI=126) in a new APM (Application Transport Message)
  • Sending PRI/APM message during tear down of CBWF call for SIP-to-SIP-I interworking scenario.

CLI command syntax:

set profiles signaling isupSignalingProfile <profile name> sendAPPinAPMforCBWF <supported | unsupported>

set profiles signaling isupSignalingProfile <profile_name> sendPRIonRELforCBWF <supported | unsupported>

If SBC is configured for CBWF and if a CONFUSION message contains Cause 99 or Cause 110 indicating an error in the APP parameter is received on the SIP-I leg, SBC releases the call.

This functionality takes precedence over the ISUP Signaling Profile flag transitCfn used to transit the CONFUSION message towards the other leg.

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