А что он кстати делает? Давно интересно уже
ConnectionService is a new API on Android that allows VoIP apps to better integrate with the system. The problem is that phone manufacturers didn't all implement it correctly and it's very much hit or miss sort of thing. Because of this, it's currently only enabled on a handful of Google devices (namely Nexus 6P/5X and all Pixels) where it's guaranteed to work as intended, but this setting allows you to enable it on anything regardless.