The emergency transmitter is a device used by heroes to request backup from the Hero Association in emergency situations. According to King, the device is still in its testing stage.
Spring Mustachio uses the transmitter to request backup during his fight with Kombu Infinity.
During the fight with Garou, Glasses uses the transmitter to call Genos for aid after Garou defeats their hero group.
One-Punch Man Manga and Anime; Chapter 21, and Episode 6
One-Punch Man Manga and Anime; Chapter 82, and Episode 23