Slack also has a howto on their help center: https://get.slack.help/hc/en-us/articles/216951758 Dev Anand On Wed, Mar 2, 2016 at 2:48 PM, Dev Anand Teelucksingh <devtee@gmail.com> wrote:
"Slack was being cheeky when yesterday it said voice chat would start testing “very, very soon”. Today the new “Calls” feature starting rolling out on Slack for desktop. It lets you start a private Slack Call or launch a conference call in a channel that anyone can join with a click. And in keeping with Slack’s lighthearted style, once you’re chatting, you can send visual emoji reactions that appear overlaid on your profile picture to others on the call."
Read rest of article which includes screenshots http://www.techcrunch.com/2016/03/02/slack-calls/
Dev Anand