Get used to it. Develop a protective strategy. Monitoring phone calls has become a national pastime. Everyone from electronic hobbyists, to shut-ins, to law enforcement, to career industrial spies is involved. The point is academic however.
All wireless telephones are basically radio transceivers.
Your voice is transmitted through the air on radio waves. You receive the person you are talking with the same way. Radio waves are not directional. They disperse in all directions. Anyone with the proper radio receiver can listen in.