Core Issue
The local voice busy out feature provides a way to busy out a voice port or DS-0 group (time slot) if a state change is detected in a monitored network interface (or interfaces). When a monitored interface changes to a specified state to out-of-service or in-service the voice port presents a seized/busy out condition to the attached private branch exchange (PBX) or other customer premises equipment (CPE). The PBX or other CPE can then attempt to select an alternate route.
Resolution
When you troubleshoot ISDN trunks, you can make B-channels busy out. Issue the ds0 busyout X command under controller configuration mode for the channel associated signaling (CAS) trunk.
Router(config-controller)#ds0 busyout X
In order to busy out Common Channel Signaling (CCS) or ISDN PRI trunks, use the isdn service b_channel X state 2 command under interface configuration mode.
For T1
Router(config)#interface serial 0:23
For E1
Router(config)#interface serial 0:15
Router(config-if)#isdn service b_channel X state 2
The valid states are 0=Inservice, 1=Maint, 2=Outofservice, and X is the B-channel number in both CCS and CAS configurations.
The show isdn service command can be used in order to find the state of each B-channel.
For more information, refer to TDM Switching of Voice and Data Calls on AS5400, AS5350, and AS5850 Gateways Configuration Example.
For related information, refer How to busy-out B-channels on a Catalyst 6608 module in Cisco CallManager for troubleshooting