Regular voice calls stopped working, was earlier. Maybe Skype related. Android 4.0.3
You have multiple options to solve this:


Go to Settings->Apps, scroll to Skype, open it, and hit the "Clear Defaults" button
If that for some reason does not work: simply install some other VoIP app, try to call a person again (now you should be able to chose), hangup, and remove the other VoIP app again
Try Default App Manager Lite, DefaultApp Reset, or similar apps to reset single default actions
Chuck-Norris-Mode: try Clear default app for action to reset all default actions
Last ressort: uninstall and re-install Skype


All above solutions should reset the default app selected. Besides: same procedure works for multiple web browsers, home screen replacements (launchers), image viewers...