Doorbell doesn't trigger video on phone ...

So just bought video doorbell. I was expecting the video feed to open on my smartphone the moment someone rang the bell, so I could initiate a conversation, but it doesn't. All I get is a sound notification. The video feed works, but only when I unlock my phone, go into the app and open the vid feed manually, by which time whoever was at the door is LONG gone. Am I doing someting wrong, or have I been misled by the advertising?