Cisco 7911 Phone Bug When Updating Its Firmware

Good afternoon, 
At the CIPT1 course I am currently teaching in Almaty, one of the participants asked me to help with a problem, related to the Cisco 7911 phone. The phone refused to accept new software (the TFTP server had SCCP 9.2.3 firmware, CUCME v7.0 was used as the TFTP server). There was a message there on phone display, saying “Upgrading” along with the MAC address of the phone. The student brought the second 7911 phone and it also behaved exactly the same way as the first one. 

Obviously, it was necessary to conduct a deeper analysis of this behavior of the phone. To do this, we decided to use Wireshark to see which packets this device sends and receives. It was found that the phone receives all the necessary settings via DHCP (including the address of the TFTP server with option 150), but did not show any further activity.
The solution was found on the vendor’s website here. The following actions have been taken:
– SCCP 8.2.2 SR1 firmware was uploaded to the TFTP server;
– when you turn on the power on the phone, press the # key and hold it until the MWI LED starts flashing:
– reset the phone to factory default with the sequence: 3491672850*#, after that the phone was completely reset;
– the phone received DHCP parameters and then pulled firmware 8.2.2 from the TFTP server;
– after downloading the firmware, the phone rebooted and successfully registered on CUCME, i.e. the phone’s performance was restored.
However, then an attempt was made to upgrade the firmware to the current, 9th version. To do it we decided to register the phone on the laboratory CUCM version 8.5, which had firmware 9.2.3. When registering our phone with firmware 8.2.2, the upgrade attempt failed – the phone contacted the server, the upgrade process was activated, but when it started, the message “Auth Fail” was displayed and the software update process was interrupted. Next, the phone was initialized with its existing firmware 8.2.2 and registered on CUCM.
The solution for the second problem that appeared was also found. It turns out that you can upgrade to the 9th firmware versions only if the phone had firmware versions 8.3 or newer. The phone was upgraded from version 8.2.2 to 8.3.5, and then, when registering for CUCM, it successfully automatically upgraded to 9.2.3.
This is how I managed to successfully restore both 7911 phones. If any of you encounter a similar problem on 7911 phones, the method described above helps to fix the situation.