This problem occurs if the average CPU load on the AS5400 is too high (greater than 89 percent). This leads to overloading.
By default, calls are dropped if CPU goes above 89%. Call Admission Control (CAC) can be used to avoid overloading.
CAC for calls handled by Tool Command Language (TCL) 1.0; Interactive Voice Response (IVR) TCL 2.0; and VoiceXML applications can be configured by setting the percentages of memory and CPU utilization that are optimal for the Cisco voice gateway and for the particular application scenario.
When the percentage levels are set, incoming calls are denied whenever the current system CPU or memory usage (or a combination of these) exceeds the resource thresholds. This denial prevents the gateway from overloading.
The default is 89 percent for the CPU and 98 percent for RAM. However, on a high-end platform such as the Cisco AS5400, resource thresholds can be increased to accommodate more calls. To configure CAC and set the optimal system CPU and RAM usage thresholds, issue the call treatment on and call threshold global commands, as shown:
Router(config)# call treatment on
Router(config)# call threshold global cpu-5sec low 50 high 80 treatment
Router(config)# call threshold global total-mem low 80 high 90 treatment
For additional information, refer to these documents: