In order to solve the problem, check your Internet connection and make sure that all the programs that may use your bandwidth are not running. After that, close the software, navigate to Start > Control Panel > System and Security > Windows Firewall > Allow a program through Windows Firewall and check the box next to Skype. If you can't find the application in the Allowed programs and features window, press the Allow another program button, navigate to the installation directory, select the executable file and press Add. After that, from the Advanced Settings select Inbound Rules > New Rule. Check the Port box, click Next, select All local Ports and follow the self-explained steps. Also, make sure that your security program doesn't block the Skype connection. If the problem persists, download and install the latest version of Skype and check the official troubleshooting guides.
Note: There are some countries like China, UAE, Oman, Syria, Turkey, Malaysia, Bahrain, Belize, Iran, Kuwait, Libya, North Korea, Pakistan, Thailand, Tunisia or Vietnam that blocked the VOIP services. If you're living in one of these countries, you can't use the software.