Linking to the same number happens at the iCloud level. Apple already has hooks in place with carriers to enable "Wi-Fi Calling on iCloud-connected devices", so

Linking to the same number happens at the iCloud level. Apple already has hooks in place with carriers to enable "Wi-Fi Calling on iCloud-connected devices", so an iPad/Mac can make/answer calls from the cellular number of an iCloud-linked iPhone (without going through the iPhone at all). The LTE Apple Watch is just another device like that. https://support.apple.com/en-us/HT203032 https://support.apple.com/en-us/HT203032