Hackaday

An analog telephone adapter (ATA) like the Grandstream HT802 can be repurposed to create a mini telephone exchange without connecting to a VoIP network. By configuring dial plans with local loopback addresses, multiple analog phones connected to the ATA's FXS ports can call each other directly. The setup involves accessing the ATA's admin interface via Ethernet, configuring dial aliases to route calls between ports, and optionally enabling auto-dial features. While not the intended use case, this approach offers an inexpensive way to establish a small, local phone system.

Create A Tiny Telephone Exchange With An Analog Telephone Adapter