In a standard web browser, you should be able to perform the following tasks.
http://<phone ip>
Should display the phone configuration
http://<phone ip>/CGI/Screenshot
You will need to authenticate here with a CallManager username. (BOC-Admin)
This should display a picture of what is currently displayed on the phone.
If you get errors, usually it is related to the Auth URL Configuration on the CallManager.
This list provides possible CiscoIPPhoneError codes:
Error 0 = Authentication Failure error
Error 1 = Error parsing CiscoIPPhoneExecute object
Error 2 = Error framing CiscoIPPhoneResponse object
Error 3 = Internal file error
Error 4 = Authentication error
Error 5 = network error
Error 6 = timeout error