If you receive an error about being offline, it means the Skype application hasn't been connected to the log-in servers of the contact you are trying to call is actually offline. The first thing to do is to restart Skype to log-in again. Secondly, access the firewall in your computer and make sure Skype is added to the exclusion list. Go to Start > Control Panel > System and Security > Windows Firewall > Allow a program through the Windows Firewall.
