The popular instant messaging app WhatsApp is testing a new feature that will make custom wallpapers visible even during voice calls. Right now, WhatsApp selects the custom wallpaper when placing a call but that all changed in a recent iPhone app update.
As you can see in this screenshot, WhatsApp shows up as the chat wallpaper when you place a voice call. At the moment, WhatsApp does not show up as the real chat wallpaper (since you can select different wallpapers for different chats) but WhatsApp always selects the default one. Note that, since this feature is under development, things may change before the release of the feature, so the call wallpaper may reflect the chat wallpaper in the future.
There is of course no telling when this change will be made available to everyone or if indeed it ever well, but we can wait and see if a future App Store WhatsApp update adds the feature in the future.