I got a CP-6921, did the firmware update to latest version, created TFTP server , transferred XLM file with setup information, was able to register the phone and make outbound calls and to other extensions.
Can´t receive any type of calls, the sistem returns 404 error when invited.
I understand this is something inside the .cnf.xml file, a setting needed to be changed maybe.
First of all, my apologies for not reading your messages and replying earlier. I have had cases where the latest firmware will not work properly. For the 7911, for example, I am still on v8 firmware because the v9 one is too buggy. A lot of the older phones have that kind of problem, but I would expect the 3905 to be better. I will take note of the SR3 not working for you and the SR2 working instead.
Try downgrading the 6921 as well, try one version at a time. Sometimes it is a case of incompatibility with the server you are using.