Cisco Unified Communications Manager (CUCM) includes a feature called "call throttling" that denies new call attempts when the system is in a state that may lead to delayed dial tone. The Real Time Monitoring Tool (RTMT) will generate a code yellow alert when call throttling is activated. Third-party network management applications will also generate a code yellow alert alarm to indicate the fact that CUCM has begun throttling calls.
If the CUCM system is in the code yellow alert for too long (a setting you can configure through the Call Manager service parameter), the system will go into code red and the call manager service will be restarted. The code yellow alert indicates CUCM is running low on one of the following resources:
High disk I/O and disk fragmentation could also lead to a code yellow situation. CUCM monitors various latency parameters to identify system trouble that can result from a variety of conditions. These parameters can be viewed and tuned in the Call Manager service parameters (System > Service Parameters). However, Cisco does not recommend tuning the CCM Call Throttling parameters unless suggested by Cisco TAC.
Call throttling can be turned off by setting the System Throttle Sample Size to zero.
Troubleshooting Cisco Unified Communications Systems (training course)
Monitoring & Troubleshooting CUCM 6.0 High CPU using Real Time Monitoring Tool (RTMT)
Call Throttling and the Code Yellow State (pdf file)
Author: Dennis Hartmann