Slack did a little headfake last week, first saying it would add native voice chat “very soon”—then offering the feature to select users the very next day. The unveiling has been widely anticipated following last year’s acquisition of calling and screen-sharing app Screenhero by Slack.

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The feature, according to TechCrunch, is for the time being a beta available to less than 50% of Slack users, and allows audio conferencing and emoji during calls. It works through Slack’s desktop app, not, apparently, through the web interface. Video calling is expected to be added soon, along with wider availability. Slack had previously offered integration with third-party conferencing and calling tools including Microsoft’s MSFT Skype and Google GOOG Hangouts, but the new integrated calling feature is reportedly much slicker.

Slack’s call integration has huge implications for its competitors in team communications—most obviously, Skype. Skype is a great voice calling app with limited text chat tacked on, and before Slack existed remote teams simply had to make the best of it. Slack’s stratospheric growth over its mere two years of existence shows how much demand there was for its tailor-made solution. For example, we at Fortune use Slack to connect remote workers. (Google Hangouts has much smaller market share, maybe because it just doesn’t work well).

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But voice calling has made it tough to drop Skype entirely, with many teams using both. Adding voice to Slack, then, makes Skype decidedly less relevant, potentially pushing it back to its humble roots as a way to get cheap international calls. That could in turn weaken Microsoft’s entire enterprise cloud strategy, of which Skype for Business (formerly Lync) is a significant part.