Given the poor audio quality of post-Communist phone systems, high priced equipment designed for Western networks may not improve the intelligibility of voice signals much more than inexpensive measures. But with a two-way conversation, the person at the station will sound much louder than the person at the far end. If the level is set high enough to make the person at the far end audible, the near-end person will be too loud. The solution, obviously, is to separate the incoming and outgoing signals so their levels can be brought into balance. That is harder than it sounds, because both signals travel on the same pair of wires.
Any station can build one at minimal cost.
It is based on the fact that in most conversations, people take turns speaking. Putting the switch in one position sends the incoming speech signal to one mix channel. Putting it in the other position sends the outgoing speech signal to another mix channel. That way they can be processed independently, then recombined in the program audio. The switching can even be automated, by putting a line-level sensor on the studio microphone's output.