I believe the 7821 have a similar configuration file as the 9951 and 9970 as I see here: https://supportforums.cisco.com/discussion/12589941/configuring-cisco-cp-7821-sip
If i’m right then this configuration template and guide should work: http://labs.wrprojects.com/configuring-a-cisco-9951-phone-for-asterisk/
Regarding the web interface, there is no web interface to configure sip parameters but this: https://github.com/xpheres/sipManagerSuite/
This configuration tool is unfinished and for the moment just allow you to see the key parameters of the config file in a form. It will allow you to change and save those parameters in the future. Even making easier to configure the config file, you still have to send it to the phone yourself with a TFTP server.
I hope it helps