Not a “voice chat”, but Briar exist for text chat, and sending images in a one to one conversation. You can also have forums and group chats. If you have like a bunch phones with brair installed, you can have a mesh network for communications, using Bluetooth. Also works via connecting thw same Wifi network or via the internet through Tor. Think of like protests, but the government shut down the internet and cell towers, with enough proters intalling Briar, you can still organize “off the grid”.