Why did Skype hijack my phone?

I installed "Skype for Business" recently (required by my org) and everything seemed fine.

Some time later, I clicked on a phone number and selected "Call 888-555-1212 using iPhone" from the contextual popup as usual - and Skype came up. Yuck.

After a _great_ deal of searching, I found that reverting to the default behavior (of actually calling with my iPhone...) is as simple as:

• Open the FaceTime app.
• Open its Preferences.
• Look for the "Default for calls" popup, at the bottom.
• Select "FaceTime".