It is possible to write a plugin to extend RhoElements. The plugin is based on the NPAPI standard.
A couple of points before attempting this.
The ET1 is easy. It's does not contain a phone application or GSM module. Shutting down the Phone Application is not recommend on windows mobile device. The Phone Application takes care of additional functionality (Depending on device, updating the signal level in the registry for state notifications for example)
One recommendation I can give. if you use TAPI within your plugin. you could for example.
Automatically hangup incomming calls.
Turn the GSM Radio off / on, whenever you like.
Using the API call AllKeys(TRUE) will block any hotkeys therefore stopping the Phone Application coming to the front.
Hope this Helps.
You can use AppCenter to block the Phone application for incoming/outgoing calls whilst allowing RhoElements to run.