How to answer whatsapp or viber call (not regular phone call ) with voice command or gesture on s5 ?

I know how to use voice command and hand gestures to answer a regular phone call on my samsung galaxy s5, but when i try using the same commands and gestures to answer internet calls on my phone through whatsapp and viber, i am unable to answer the call. Can someone suggest me a solution for this.

Those gestures and commands only work with Samsung’s apps.

You'd probably have to write an app that you can run by voice, that can detect which app you want it to run and what you want it to do with that app.

So practically, no, you can't.