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.
Cisco recommends that you have knowledge of these topics:
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.
Technical Tips Conventions for more information on document
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
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
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
Ring back tone—When you transfer calls to the SIP client from a SCCP
Announcement—Devices configured for Cisco Multilevel Precedence
Barge tone—Before a participant joins an ad hoc conference
for more more information about annunciators.