The native dialer is the phone's operating-system-level Phone app: iPhone's "Phone" app, Android's "Phone" app, equivalents on other platforms. It handles all real cellular calls, integrates with the lock screen, registers with CarPlay and Apple Watch, supports system-level call blocking and screening, and works whether the phone is locked or unlocked, foreground or background, with any other app open or no app open.
VoIP and softphone business numbers don't ring on the native dialer — calls ring inside the VoIP app, which means the OS has to keep that app's listener alive. Modern operating systems suspend background apps aggressively to save battery, so VoIP softphones can miss calls because the OS killed the app, not because the user didn't pick up.
A real cellular line — like a Kelir business eSIM — rings on the native dialer the same way a personal cell line does. There's no app dependency.