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Overview

The SBC Core 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 the 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>


The SBC, Ribbon 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 Ribbon 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)


Figure 1: SIP Protocol Interworking Using CBWF



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

Table 1: 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

The SBC Core 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 the 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, the 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.