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With Cisco CallManager and MGCP gateway, some inbound calls from PSTN through T1 PRI are not routed inside. A fast busy signal is received


Core Issue

If you check the Cisco CallManager trace, the Telco is sending full number (en bloc mode), while CiscoCallManager is sending back the SETUP_ACK, waiting for more number (overlap mode).

Cisco routers can receive an ISDN call in en bloc mode or overlap mode. When configured    for en bloc, the setup message should contain all necessary addressing information    to route the call. In overlap, the setup message does not contain the complete    address. By default, H.323 is running on en Bloc mode but with Media Gateway Control Protocol (MGCP), it runs in verlap mode.


To resolve this issue, turn off the Overlap Receiving Flag for PRI in Cisco CallManager by performing these steps:

  1. Open up the Cisco CallManager Administration.
  2. Go to Service > Service Parameters.
  3. Select Cisco CallManager from the Service pulldown menu.
  4. In the Service Parameters Configuration page, look for the name for Overlap      Receiving Flag for PRI.
  5. Change the parameter from True to False.
  6. Click Update.

Note: The Overlap Receiving Flag for PRI parameter indicates whether overlap receiving is enabled for all PRI interfaces that use the PRI backhaul mechanism. Valid values are True (overlap receiving is supported) or False (overlap receiving is not supported). Default value is True.

To resolve this problem, upgrade the Cisco CallManager to version 3.3(4) or 4.0(2a). Refer to Voice Software.

For more information, refer to these documents:

Problem Type

Call control software  (CallManager, CallManager Express, ICS7750, SRST, SS7 call agents)

Call connection  (Connection quality: One-way audio, no busy tone, dropped calls, etc.)

Call Control


Call Connection

Busy tone

Reorder tone

CallManager Features

Bad operational behavior, status,information, performance, delay or other CCM failure

Call routing

Failure Type

Nothing transmitted: Not sent or received, notification, alert or delivery, etc.

Cannot connect to server, disconnect, register, synchronize, communicate, etc.

Bad operational behavior

Frequency and Scope of Symptom

Occurs only on some calls

When Problem Occurs

After last digit is dialed

Call Topology and Direction

PSTN/PBX/gateway > IP network

Signaling Protocol


TDM Signaling Type




MGCP Features

MGCP gateway

CallManager Versions

CallManager 4.1

CallManager 3.0

CallManager 3.1

CallManager 3.2

CallManager 3.3

CallManager 4.0

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