Trying to figure out the ultimate cause of this frustrating symptom led us down a dark tunnel that, at times, seemed to have no light at the end. If you can guess the final answer before I reveal it, you've earned a Bugs & Fixes gold star.

The proximate cause of the busy signal was easy to locate. We navigated to the Privacy section of Skype's Preferences. The options to "Allow calls from" and "Receive calls to my Skype Number from" were set to "Contacts" rather than "Anyone." This meant that anyone not in my friend's Skype Contacts list (which was empty at the time) could not call her. Apparently, when such calls are blocked, the caller gets a busy signal.

I would have appreciated if Skype provided more feedback when a call was blocked, but at least we now knew what needed to be done. Or so we thought.

We changed the appropriate Privacy settings to "Anyone." Success! Our next test call went through.

Unfortunately, the busy signal symptom returned if we quit Skype and relaunched it. When we checked Skype's Privacy settings, we found out why. The relevant options had reverted back to the prior "Contacts" setting. This continued to happen every time we changed the settings. Uh-oh.