IANAL, but my understanding is that in Europe on-call can fall under working time, depending on the exact nature of the requirements. In fact, I'm very interested whether anyone has firsthand experience arguing developer on-call falls under such category. E.g. being 8 minutes away from a work machine and network access is not too dissimilar to a firefighter being 8 minutes away from the fire station.
https://www.eurofound.europa.eu/en/european-industrial-relat...