Tinitus wrote:
If this behavior is by design, it works since some days buggy and didn't worked for the last years. 😕
It's a feature that was added with iOS 8.3, so that's why it didn't happen in years past. And it only works when the affected devices are connected to the same WiFi network, so that's why it doesn't happen all the time. The idea is that if you are using your iPad or your Mac and a call comes in to your phone, you can answer it on the device you are currently using. See: Get help using Continuity with iOS 8 and OS X Yosemite. It also works for texts.
At the same time this feature was added, Apple created Family Sharing, so families no longer have to share the same Apple ID to share music and apps. Family Sharing - Apple Support