Cisco 7900 IP Phones do not receive a dial tone from Cisco CallManager and instead receive a reorder/busy tone when the phone goes off-hook. The High Traffic Try Again Later error message appears on the LCD display of the IP Phone.
The High Traffic Try Again Later error message that appears on the IP Phones indicates this issue:
Whenever the LowPriorityQueueThrottlingFlag Cisco CallManager service parameter is set to TRUE and the Low Priority queue is over the limit specified by the LowPriorityQueueThrottlingMaxCount Cisco CallManager service parameter, the High Traffic Try Again Later error message appears on the LCD display of any IP Phone that is registered to this Cisco CallManager server and that attempts to go off-hook and make a new call. By default, this message appears on the phone if the Low Priority queue gets deeper than 20 signals.
Generally, the Cisco CallManager works fine as long as the Low Priority queue does not get higher than 100. Therefore, if you never see the Low Priority queue go higher than 75, you can configure the LowPriorityQueueThrottlingMaxCount = 100 Cisco CallManager service parameter in order to overcome this issue.
It is also recommended to check the CPU utilization and the disk space usage. If they are high, you need to clear the hard disk space and shrink the CDR/CAR database.
The information I provided previously, was for CUCM 3.x. So, that being said the Service Parameters doesn't exist anymore. My apologies for the miss-information. After looking further into your issues, I found that the main culprit for this issue is either high CPU utilization or a configuration issue with QoS.
High CPU - You may want to consider rebooting your CUCM servers, if you are experiencing high CPU utilization.
QoS - Make sure that you're trusting the DSCP values on your switchport trunks and that you're marking your RTP and Signaling packets correctly.
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