This is not a new issue, but it comes up often enough that searching for the exact version each time has started to irritate me…  This post is actually more for my own benefit, haha!

Older phone firmware routinely gets stuck when upgrading from phones connected to older CUCM systems.  This requires a step-upgrade process to get the phones to mid-level version, followed by a jump to the desired version.  There are several occasions where this happens, but this is by far the most common.  I will extend this post when I come across more again.

Taken directly from the Cisco documentation:

Firmware Upgrade Issues

For all SCCP and SIP firmware upgrades from firmware release versions earlier than 8.3(3) to version 8.5(2)SR1 or greater, you must first upgrade your firmware to version 8.5(2). Once you have upgraded to version 8.5(2), you can upgrade your IP Phone to version 8.5(2)SR1 or later. Refer to the Firmware Versions in this Release section of this document to determine the firmware load provided in this SU.

For additional details, Firmware Upgrade Instructions, and Firmware Download locations please see:

 

SCCP http://www.cisco.com/en/US/docs/voice_ip_comm/cuipph/firmware/8_5_2/english/release/notes/7900_ 852SR1.html#wp57602

SIP http://www.cisco.com/en/US/docs/voice_ip_comm/cuipph/firmware/8_5_2/english/release/notes/7900_ 852SR1.html#wp147343

Reference:

https://www.cisco.com/web/software/282074295/142115/cucm-readme-1052su7.pdf