First let me make sure I understand your question. You have a softphone that is like Skype, and you want to use the Calisto 800 device with your soft phone.
There are 2 ways to do this.
1. Do it as a plugin to Spokes. The plugin is in C# and you add your plugin to PURE.xml. Set the default softphone to your softphone. Since Spokes and your softphone app are 2 separate processes, Your softphone app must provide an interface that allows inter-process communication. See the sample code SkypePlugin from the Spokes SDK.
2. Add the device handling code to your Softphone code in C++. Follow the SpokesSDKComSample and also refer to the document SpokesSDK.pdf, chapter 4.4. In this case you don't need to edit any xml files.
In either case make sure the Calisto 800 is the only Plantronics device plugged in to your PC, otherwise the device enumerator will make the first Plantronics devices it detects as the active device. THis may or may not be the device you want.
I hope this helps.
Wei is right, with the latest version of Spokes 2.7 you don't have to manually edit the UserPreference.xml.
If you run your compiled SpokesSDKComSample based application, and then run Plantronics Control Panel (that comes with Spokes Software* from http://www.plantronics.com/software/) you can then select your application as the default softphone - it will be listed with the Plugin Name you registered in the code.
*Note: to try this you may have to uninstall SDK first, or use a different machine, as you can't have both Spokes Software and Spokes SDK installed at same time (since Spokes 2.7). The intended use case is for selecting the default softphone on the end user PC.
For example, look at the picture below. The Plugin Name is "COM Plugin". With this running the Control Panel shows it as an available default softphone. Once I select it and click OK in the Plantronics Control Panel, the Calisto 800 series device and Calisto P240 devices will be able to dial and the SpokesSDKComSample application will receive "CallRequested" events containing the number dialled.