DHCP Scope was fine. As per your request i dig RTMT logs. Reason code is 13. cucm is 11.5 CUCM firmware version is lower than the 8841 phone firmware. so upgrade status is showing failed. from reason code i conclude its because of firmware mismatch only. but my question is why its like that. phone firmware is latest compare to CUCM device defaults....   https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucm/err_msgs/11_x/ccmalarms1151.html   Error Message %UC_PHONE-4-DeviceImageDownloadFailure: %[DeviceName=String][IPAddress=String][Active=String][InActive=String][FailedLoadId=String][Method=Enum][FailureReason=Enum][Server=String][AppID=String][ClusterID=String][NodeID=String]: Device firmware download has failed Explanation   Cisco IP phone failed to download its firmware load (image) for the reason defined in this alarm Recommended Action    Verify that the IP address or hostname of the image download server (either the Load server or the TFTP server) is correct. If you're using a hostname, verify that the Domain Name Server (DNS) is accessible from the phone and can resolve the hostname. Verify that the TFTP service is activated and running on the Load server or TFTP server (the server you are using to serve firmware load files). Verify that the Load server or TFTP server is accessible from the phone. Also, refer to the reason code descriptions for recommended actions.     Method - Method used for downloading the device firmware   FailureReason - Reason that the firmware load has failed to download Enum Definitions - FailureReason       13 The device has exceeded the internally-configured time allowed for a response from the Load server or TFTP server when requesting the firmware load file. It's possible that congestion is causing a delay in TFTP response. To allow the phone to attempt the download again, wait a few minutes then reset the phone. The phone will attempt to download the file again. If resetting the phone does not solve the issue, restart the Load server or TFTP server (whichever server provides the firmware load files).   I'm planning to do the task as mentioned in the below step: https://community.cisco.com/t5/collaboration-voice-and-video/upgrade-downgrade-firmware-of-specific-ip-phone/ta-p/3137526   but my question is whether the device default will change firmware if i upload only the new firmware and not cop file.  then will  CUCM again go for old firmware to downgrade??? im now trying for only one phone for time being until the problem again pops up.  but why all these phones at a time trying for firmware upgrade when they are already in registered state. will the cucm tell phones to upgrade or downgrade ? if so when and where we can find that? ... View more

Is this because of DHCP scope? if static IP of CUCM is in the range of DHCP scope there is chance of this issue right? ... View more

why only for phones have this issue if it is DHCP release issue. why not other dhcp devices??? if its dhcp issue then why its working while restarting the cucm server? ... View more

If I reboot secondary will it impact primary? we cannot have downtime for primary. ... View more