You need to edit the file named "3CXWin8Phone.user.config", adding the name of your DLL to the following entry

Located at: C:\ProgramData\3CXPhone for Windows\PhoneApp\3CXWin8Phone.user.config
This line needs to be present for our software to be loaded:
<add key="CRMPlugin" value="CallTriggerCmdPlugin,DotNetScriptPlugin,BusylightPlugin" />
The 3CXPhone logs located at "%AppData%\3CXPhone for Windows\Logs" will tell you what addins are being loaded.

Please note that this requires that 3CX has a PRO/Enterprice license key activated, otherwise your plugin will be ignored.