How can I create a two-way voice communication system that works over the local network?

I wish to create a simple two-way voice communication system between two Linux computers running Debian-based distros (probably two Raspberry Pi Zeroes). Each computer will have a microphone and speaker attached as well as a physical button. Both computers will be headless after setup and always-on. The computers would both be on the local network. Essentially, microphone input would need to be sent in real-time over the network by each computer and played by the other computer. The system would function as follows: When the button on either computer is pressed (let's say it is called Computer 1 in this case) Computer 1 will 'call' the other computer (Computer 2). Computer 2 will ring. When the button is pressed on Computer 2, the 'call' will be answered and voice communication will begin, with a person on each computer able to hear and speak to the other via the speakers and microphone. When either user presses the button on their computer, the 'call' will disconnect and voice communication will end, with the two computers going to their starting state, ready to accept a 'call' or make one when the button is pressed. How can I achieve this behaviour? Any solutions, including pointers towards relevant software are appreciated.