This document describes how to resolve the absence of a ring back tone when a Session Initiation Protocol (SIP) call is transferred over the Public Switched Telephony Network (PSTN) through a Cisco IOS® gateway to a Skinny Client Control Protocol (SCCP) IP phone.
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The information in this document is based on Cisco Unified Communication Manager 5.x.
The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, make sure that you understand the potential impact of any command.
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For SCCP initiated blind transfers, Cisco CallManager needs to generate tones or a ring back after a call is already connected. In other words, Cisco CallManager provides a media announcement for blind transfers. Blind transfers initiated from an SCCP IP phone allow ring back to the original, connected SIP device user. In order to accomplish ring back, Cisco CallManager uses an annunciator software device that is often located with an Media Transfer Protocol (MTP) device.
A ring back tone is not heard for a call from PSTN through a SIP trunk to the SCCP IP phone when it transfers to another destination.
In order to resolve this problem, complete these steps:
Add an annunciator to the Media Resource Group List (MRGL) of the particular SIP trunk through which the call originates.
Reset the MRGL.
An annunciator is used for SIP ring back. The SIP trunk needs to have an MRGL that contains the annunciator, or the annunciator needs to be in the default list.
Cisco CallManager uses SCCP messages in order to establish a Real-time Transport Protocol (RTP) stream connection between the annunciator and the device. The annunciator plays the announcement or tone to support these conditions:
Ring back tone—When you transfer a call over the PSTN through a Cisco IOS gateway. The annunciator plays the tone because the gateway cannot play the tone when the call is active.
Ring back tone—When you transfer calls over an H.323 intercluster trunk
Ring back tone—When you transfer calls to the SIP client from a SCCP phone
Announcement—Devices configured for Cisco Multilevel Precedence Preemption
Barge tone—Before a participant joins an ad hoc conference
Refer to Annunciator for more more information about annunciators.