Before you can "ring" somebody in their browser, their presence has to get established first. So, they need to have signalled their presence to a signalling ser

Before you can "ring" somebody in their browser, their presence has to get established first. So, they need to have signalled their presence to a signalling server first. That can happen via loading a specific URL, or being logged in to a specific Website. It's not possible to just magically know that they are around without them announcing it.