If you're using a Mac, the default calling app is FaceTime. Here's how to change the default calling app on your Mac:
Open the "FaceTime" app on your Mac.
In the menu bar at the top of the screen, click "FaceTime" and select "Preferences."
In the "Preferences" window, click the "Settings" tab.
Next to "Default for calls," click the drop-down menu and select the app you want to use as your default calling app.
Once you've selected the app, close the "Preferences" window.
From now on, when you click on a phone number or contact in your web browser, email app, or any other application, the call will automatically be routed through the app you selected as your default calling app.
If that doesn't work, here's an app for it: http://www.rubicode.com/Software/RCDefaultApp/
You have to change the "tel:" handler.
You can find more explanation on this here: https://superuser.com/questions/548119/how-do-i-configure-custom-url-handlers-on-os-x