The ability to put a call on hold and answer another call is built into the default phone apps on Android phones and iPhones.

In its current implementation, whenever you answer a video call on WhatsApp, the call defaults to your front camera. If you're, for any reason, not interested in showing yourself on camera, you ...

WhatsApp beta version (2.25.7.3 for Android), as pointed out by Android Authority, reveals a countermeasure — one that will give you the option to answer a video call with your camera off.