A Discover Call sounds when a Union activates their Drive. Normally, only other Unions can hear a Discover Call. Es can hear them thanks to a detection system coded into her while Touya Kagari can hear them for unknown reasons, likely because he is Origin of the Grimoire. Additionally, while Discover Calls are usually only hearable within 10 meters, Touya is able to hear them from up to 300 meters away.
Discover Call (Existence Resonance)
A Discover Call is a bell-like chiming that sounds when a Union uses their Drive. Generally only other Unions can hear it. Discover Calls are normally only detectable from a distance of ten meters, but for some reason Touya is able to hear them from a distance of three hundred meters 
- XBlaze Code: Embryo, TIPS