"We take outages like this really seriously and apologise for the inconvenience users are having," Tony Bates, Skype chief executive officer told BBC News.
"Right now it looks like clients are coming on and offline and sometimes they are crashing in the middle of calls. We are deep in the middle of investigating the cause of the problem and have teams working hard to remedy the situation," Mr Bates said.
On Skype's Twitter account, the company said their "engineers and site operations team are working non-stop to get things back to normal".
Mr Bates told the BBC that normal call volume for the time of day would be 20m. "Skype is one of the key applications of the modern web," he said.
"It is already a hit with consumers, and over the past few years it has become part of the economic fabric for startups and small businesses around the world. I am not sure we can comprehend the productivity cost of this outage.
"The outage comes at a time when Skype is starting to ask larger corporations for their business. If I am a big business, I would be extremely cautious about adopting Skype for business, especially in light of this current outage," added Mr Malik.