Core Issue
In some cases, Cisco IP phone users or Plain Old Telephone Service (POTS) users connected to a Cisco IOS® gateway may not hear a busy tone when they call a busy number on the Public Switched Telephone Network (PSTN). This is a problem when the PSTN disconnects the call with a normal call clearing cause code, instead of user busy. Instead of setting the cause code to user busy, the PSTN switch sets the progress indicator information element on the DISCONNECT message to eight, indicating that inband information is available. The disconnect message, however, causes the gateway and Cisco CallManager to release the call.
Resolution
Issue the voice call convert-discpi-to-prog global configuration command, available in Cisco IOS Software Release 12.2(1). Doing this converts a DISCONNECT message with a progress indicator value of eight to a PROGRESS message with a progress indicator value of eight so that audio is cut through and the Cisco IP phone hears the disconnect tones or announcement from the PSTN. For further information, refer the document Troubleshooting No Busy Tone and No Announcement Messages on ISDN-VoIP (H.323) Calls. For more details on troubleshooting this and various other issues refer to The Cisco Press book Troubleshooting Cisco IP Telephony [ISBN 1-58705-075-7].
Problem Type
Call connection (Connection quality: One-way audio, no busy tone, dropped calls, etc.)
Call Connection
Dead air
Disconnect tone
Call connection establishment
Disconnection failure
Announcement
Busy tone
When Problem Occurs
While call is connecting
End Where Problem is Heard or Device Type Involved
IP phone
Call Topology and Direction
IP network > PSTN/PBX/gateway
Q931 and H225 Call Setup Messages
DISCONNECT
PROGRESS
Common Q931 Information Elements
8 - In-band information or an appropriate pattern is now available sent in alert
80 - Normal Disconnect
Progress Indicator Alteration
voice call convert-discpi-to-prog
