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 Briar installed, you can have a mesh network for communications, using Bluetooth. Also works via connecting to the same Wifi network or via the internet through Tor. Think of use cases like protests, and the government shut down the internet and cell towers, with enough protesters intalling Briar, you can still organize “off the grid”.