I'm using a single board computer to talk on a phone line through two coils, one for the TX part, the other for the RX. The phone line is on another coil and then it's connected to a two-wire circuit that leads to an handset. As you know, the problem with this config is that if you build a full-duplex communication system with VoIP in the middle, you hear the echo from what you just transmitted on the network coming back See this connection scheme
Is there a circuit which is able to reduce or eliminate such echo? Right now I'm using a DSP to implement a simple subtractor (Rx=Rx-Tx), but maybe this common problem has been solved by clever analogue engineers back in the past.