This is a simple homemade intercom (also called intercommunication device, talkback or doorphone). It is enough loud, you can reliably hear the voice several meters away from it and therefore you can use speaker instead of handset. Loudspeakers are used also as microphones for simplicity. Amplifier and the power supply is only on one side, switching is done using a DPDT relay a (SPST) switch. Of course you can omit the relay and use DPDT switch. I use relay because DPDT switch is harder to come by. Normally, the relay is switched to receive position, but on the remote side the switch is closed, as it is the Normally Closed (NC) type od switch. Thus the small resistance of speaker coil grounds the base of first transistor. It is therefore closed, thus the second transistor is also closed and no current is flowing. Only a very small current flows through the 1M resistor, so the device can be permanently connected to the battery. 9V battery should last at least one year. After pressing the NC button on the remote side, the speaker is connected through a capacitor, base is no longer grounded, both transistors are opened and they act as an audio amplifier. By pressing the NO button the relay swaps the speakerd functions. Speakers should have an impedance of at least 32 Ohms. Remote side has no semiconductors and other polarized components, and therefore you don't have to observe the polarity of the cable. If the cable should be long (more than about 20 to 30 meters) it would be appropriate to use thin coaxial instead of an ordinary pair to prevent the mains hum getting into the amplifier. If no switch is pressed, both transistors are closed and the speakers are completely silent.