I have a Windows Phone with 8.1 installed and a family member recently got one as well. I've never used Skype, but thought we'd give it a try. When we're on a call, there's a button on the screen for Skype on both of our phones but it is "grayed" out on both phones. I've made sure I'm signed into Skype and the other person is signed into Skype, but the buttons are still grayed out. What am I missing?