Well, I have finally done it, with 2.5 days time spent, a couple hours on telephone with two Cisco engineers who were both wondering themselves why this thing was behaving the way it was.
The two phones I purchased had "Application Load ID (AKA: firmware) of “P003AM30”. This is their “skinny” protocol load. If you’re trying to do sIP, you need a load that starts out “POS…”. You can not upgrade from “P00…” to “P0S…”, you need to downgrade to “P0S30203” to get it using “POS…” firmware, then you can upgrade to the newer releases of the SIP firmware, with one extra thing to know.
You do not need to step through every version of he firmware, you can jump versions of firmware, but what you encouter is the issue with their “signed binaries” (ie: “*.sbn” files) that they have converted to.
If you have both a “.bin" and a ".sbn” file in the TFTP server root directory, it will default to loading the “*.bin” (ie: unsigned binary), which you do not want to do, since you need to convert over to signed binaries, in order to continue upgrading to get to the higher versions which only come signed. If you try to load higher version binaries that are not signed, the phone will fail to load and give an error as such (which I dont have the exact verbiage of).
So, bottomline, go down to SIP 2.3, then go up to the first signed binary, then go to the final signed binary, then you ought to be there.
Their documentation states nothign about any of this. I asked my engineer to pass along my request to have that corrected & he said he would.