Android based VoiP only mobile device

I read recently how someone built a VoiP only mobile phone using an iPod so it never connected to mobile towers and operated entirely off public, home and mobile WIFI. It sounds really good except its based on iOS and requires an Apple account.

Now I love my Nexus 6P running Copperhead and wondered whether I could build something similar to the iPod mobile VOIP device but based on Android, ideally something like Copperhead so no need for a Google account either.

So it would be small and mobile like a mobile phone but it wouldn't be based on phone hardware so never even attempts to connect to cell towers but would otherwise work using any Android VoiP apps.

Has anyone ever done something like this and could anyone suggest hardware to look at as a basis for such a device?

Thanks in advance for any suggestions