First of all you can not configure any Cisco IP phone from by going to its web page - you can either do it from a call control server which push the configs to phone or have to edit the xml file and then supply the files to phone via tftp.
If you have got call control as CUCM - phone config is done in CUCM, web access to phone can be enabled from CUCM device page- change the Web Acess parameter to enabled if its not.
If CME is your call control - you will configure the phone from CME and can enable web access from CME under the Telephnoty-service (you can google for exact commands)
If its any free IP PBX: And you are looking to set some parameters, you will need to modify XML file.
Ref: http://www.voip-info.org/wiki/view/Asterisk+phone+cisco+79x1+xml+configuration+files+for+SIP
-Terry
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