Where can I find a 2-way intercom type of device that is always on and allows each end to talk simultaneously without pressing a talk button?

I'm looking for a device that would allow 2-way communication between two devices that is always on/active and doesn't require your to press a button to talk Basically looking for something that provides a live open communication line such as a phone line where both parties could essentially talk simultaneously. Everything I'm finding seems to require a press to talk 2-way intercom type of solution.