When you are currently on a voice call with your mobile device you cannot start video on demand by pressing "Watch live" or answer a SkyBell call at the same time. This is due to the way the SkyBell HD interacts with other media streams such as voice calls. Two media streams can not be established at once on Android or iOS devices because the SkyBell HD's video stream uses specific ports for the best possible video consistency in 1080p.
You will see a push notification that someone triggered the motion sensor or pressed the button and if you want to connect with that person at your door you'll need to end your voice call by hanging up, open your SkyBell HD app and press "Watch Live".
SkyBell V2 Classic
When on a voice call with your mobile device you can on demand "Start Video" while you are on a call but the microphone will be engaged with your call and will not play sound from your SkyBell.
This is because the microphone being used on a phone call is the same microphone being used on the SkyBell Classic app and the phone can not provide the voice to both streams through the microphone.