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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
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:
Open up the Cisco CallManager Administration.
Go to Service > Service Parameters.
Select Cisco CallManager from the Service pulldown menu.
In the Service Parameters Configuration page, look for the name for Overlap Receiving Flag for PRI.
Change the parameter from True to False.
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.