A problem with Skype's status system has meant many people arecurrently 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."
"We're in the process of reconnecting our users, and focused on restoring full service. The issue did not affect Skype for Business users," it said.
It also apologised for the inconvenience being caused.
Several users contacted the BBC to discuss how they had been affected.
"What is annoying me is that my eldest son is far away in Japan and Skype is our main method of contact," said Howard Wensley from Blackpool.
Florence Morimon from Norwich added: "I am a freelance translator and French tutor so this kind of problem affect directly my business. I had to use Google Hangouts to make a video call this morning, but the quality is not as good."