This tip provides guidelines and best practices on how to upgrade the 7900 Cisco Unified IP Phones from firmware version 8.3(x) to version 8.5(2) or later so they can successfully register on a Cisco Unified Communications Manager Business Edition 3000 server.
You cant upgrade from 8.3.2 on third gen phones to 9.2.1. You need to first go to 8.4.4 or anything between 8.3.3 and 8.5.2.
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.