A week ago, Slack's CEO Stewart Butterfield said Slack intended to provide a way to integrate directly with Microsoft Teams calling. Today, April 1, Slack is rolling out a beta of the Microsoft Teams Calls app for Slack.
Slack also is unveiling additional VOIP phone integrations in addition to the Teams one. It is integrating Slack with calling services from Zoom, Cisco Jabber, RingCentral and Dialpad. These integrations are all designed to allow users to launch calls to phone numbers directly from within Slack, Slack officials said.
Butterfield didn't say last week how and when Slack intended to provide this integration with Teams calling and chat. Later, both Slack and Microsoft spokespeople told me that Slack would be doing this via publicly available Microsoft 365/Office 365 programming interfaces.
Slack already provides a number of integrations with various Microsoft Office applications, including Outlook, OneDrive and SharePoint. The beta of the new Microsoft Teams app is available here.
During its fourth quarter FY20 earnings, Slack officials said that the "majority" of Slack's $1 million and above customers also use Microsoft's Office 365, the commercial versions of which includes Teams for no additional cost.
Microsoft recently disclosed that Teams currently has 44 million daily active users. Slack officials have declined to say how many daily active users Slack currently has; the last figure it shared publicly in October 2019 was 12 million.