Speed or duplex mismatch on ethernet ports. Disable auto-negotiation of these parameters and hard code 100 full duplex on both sides.
High CPU utilization on CallManager. Check CPU utilization on the Cisco CallManager server through the Windows Task Manager. Select Ctl Alt Del then the Performance tab. Check which process is causing the high CPU utilization by selecting the Processes tab.
Note: Although 100 percent CPU of a high-level process can cause sluggish behavior or delayed dial tone, do not infer from this that 100 percent CPU is necessarily always a bad thing. As of CallManager 3.3(1), low priority tasks (such as phone registrations) can consume 100 percent CPU without causing adverse effects to the ability to place or receive calls. Look at the 100 percent CPU as a possible symptom but not necessarily the root cause. In this case, you observe the symptoms of sluggish or delayed dial tone and 100 percent CPU utilization and make a correlation between the two.
To investigate CPU spikes more thoroughly, make use of the Real-Time Monitoring Tool to view the counters in the Cisco CallManager System Performance object to view the status of CallManager system performance objects. Counters available detail which queue level the usage is attributed to, so even when the CPU spikes to 100 percent utilization, you can understand where the spike is occurring and determine whether it results in degraded CallManager performance.
High memory utilization and disk fragmentation on Cisco CallManager. To reduce the impact of memory paging to the hard disk, regularly check for fragmentation on the CallManager volume, particularly if tracing is operational. If the total fragmentation appears to be more than 10 percent or if files have become overly fragmented (more than 500 fragments), it is advisable to defragment the volume.
High bandwidth utilization of remote link between Cisco CallManager and IP phone. Ensure that low latency queuing and Connection Admission Control (CAC) are correctly configured.
For more information, refer to the Call Processing chapter of the IP Telephony Solutions Reference Network Design (SRND) Guides. These design guides are available for difference CallManager releases. This includes detailed information on Cisco CallManager performance, including the CPU and memory consumption of various IP telephony devices.