You can use Voyant Connect for making calls using Callto and Tel URL links anywhere in your machine, but you need to register Voyant Connect as the primary application for that purpose in the operating system.
On Windows 10, at first login there is a dialog asking if you would like Voyant Connect to be the primary calling application for Callto and Tel URL schemes. Answering yes will modify Windows settings so that the next call using those links will use Voyant Connect. If another application should afterwards take precedence in Windows settings, you can again make Voyant Connect the primary calling application from Preferences, Outgoing Calls tab.
On OS X, you need to make the same selection in FaceTime preferences. You are required to log in to FaceTime to change the preferred calling application.