In a case of hell freezing over, the two biggest names in the workplace chat market may be about to unite in a way few predicted they would. CNBC reports Slack is working on a way to allow cross-platform calling with Microsoft Teams.

If you’re unfamiliar with why this is massive news, here’s a refresher. Microsoft Teams and Slack are the two leaders in collaborative communication. They also happen to be bitter rivals. Teams arrived after Slack and quickly surpassed it in terms of users.

Amid the rise of Teams, Slack has enjoyed consistent growth and criticized Microsoft for not playing fair in the market and lamented the fact Teams is bundled into Office 365.

So, the idea of Slack working on integration with Teams is surprising. But then, these are surprising times. The ongoing COVID-19 pandemic has resulted in many nations going into quarantine lockdowns, with only essential workers allowed outside. Tens of millions of people around the world are now working from home.

Teams and Slack Growth

We have seen how both Microsoft Teams and Slack have benefited from this situation. Those millions of at-home workers need to leverage both services for inter-organization communication.

This week Slack had a record 12.5 million concurrent users on its platform. Microsoft announced last week Teams has seen its daily active userbase double in the space of weeks, up to 44 million.

If any other two companies was going to work on cross-calling integration, it was likely to be Slack. The company already plays nice with other Microsoft services, such as Outlook, OneDrive, and SharePoint.