You half answered your question. The other 1/2 of the answer is this.
Since pretty much anything is legal in your country (where are you, way out on some island? LOL) what you do is let someone use the phone, and as soon as they are done speaking, take the phone away from him/her and listen to the other guy.
OR
cut the phone wire and observe the connections. If there are three wires, one is for the listening speaker, one is for the microphone and the other is common to both. Rip your own phone hand set off your phone, and somehow hook it up to the pay phone, and now you can interrupt the caller as well as listen to what the caller has to say.
BUT, if you are not stupid, then please GIVE UP