A problem with Skype's status system has meant many people are currently unable to use the service.
Reports about Skype being unavailable started at about 09:00 BST on Monday morning in the UK.
In a statement, Skype-owner Microsoft said it had "detected an issue with the status settings" on Skype that show if someone is online or not.
The problem locked many people's status as offline - stopping them making calls even though they were connected.
In addition, said Microsoft, all the contacts of affected users would show as offline meaning they would not be able to call those people.
Many Skype users in the UK, Australia and Japan have all reported problems.
Skype support staff acknowledged the issue in a tweet and said the firm was working on a "quick fix".
In a lengthier statement put on its blog, it said: "The status issue also doesn't affect Skype for web, which can be used to make calls and send instant messages."
It also apologised for the inconvenience being caused.