In 2022, Major League Baseball first allowed PitchCom, a system that allows a catcher to call signs by pressing a button on a transponder attached to his arm or shinguard. The signal is relayed to the pitcher via a microphone stored in his cap above his ear, so the pitcher can audibly hear what pitch the catcher is calling.
This year, MLB is allowing teams to reverse the process, with the pitcher having the transponder on his person and relaying the sign to the catcher. This allows the pitcher more control, and it also makes the process a little easier in a loud stadium, because the catcher can wear an actual earpiece rather than something near his ear.