Skype

 Skype  is a communication service that provides text chat, and audio/video calling service via native applications on desktop (Mac, Windows), and mobile (iOS, Android), and was acquired by Microsoft in 2011.

You can purchase "credit" on Skype and use it to make audio calls to telephone numbers.

Microsoft account doesn't exist
If you try to login with your email address in a Skype native app or https://skype.com/ and get the error message: "'That Microsoft account doesn't exist. Enter a different account or get a new one.'" then you might be able to recover the account using their diagnostics UI: Try entering your email there, and if it can find your account, you should get a message like: "'You have 1 Skype account(s) for [your-email]. Select your account then sign in or reset your password.'"
 * https://a.login.skype.com/diagnostics

Following this process I (undefined) was able to recover my Skype account on 2017-09-05 that I had not used for maybe over a year (last email notification from Skype was dated 2016-08-02).

Lacks skype URL support on Fedora
Apparently on Fedora Linux, even when you install the Skype app, click on "skype:" URLs fails to launch the Skype app.. This issue is present on the latest versions.

Basically, installing the Skype app does not automatically configure a handler. This would mean you would either have to script or manually add this functionality. And this would differ based on the Window manager.

Credit deactivation
If you don't use your Skype credit for 180 days, it becomes inactive.

"Once it becomes inactive, you can reactivate it whenever you're ready to use it. Simply sign into your account online and follow the option to Reactivate credit." - per email received by Tantek Çelik on 2014-04-29 with subject "Your credit will become inactive in 7 days".