On Slack, you need to wait for your colleague to accept the call. We want to give users the option to be in an "active" state, where they immediately can talk to each other. If I am working on a project in XCode for example and want to ask my coworker about an idea i have, I don't have to switch apps, search the colleague, press call and wait for them to answer. I just move the mouse to the menu, see that they are active and I am instantly connected