Although the function Continuity is not new (from iOS 8), and especially for making phone calls between iPhone and iPad via Continuity is quite basic but very handy so I want to repeat and guide the setup for you I am using iPhone and iPad in parallel but never tried.
Note: iPhone and iPad must use the same Apple ID account.
Go to settings (settings) – Phone – call on another device (Calls on Other Devices) – push the allow button (Allow)
If you own multiple Apple devices with the same Apple ID, you can turn off / open any device that you want to allow calling.
It's done! From now on you can open a call from your iPad without clicking on the iPhone if you're working and using the iPad in your hand, which essentially uses calls from the iPhone the way we call on the Apple Watch: From the iPad – select contacts and press normal call (phone icon next to FaceTime button)
Of course the call is made via external speakers.
Note: FaceTime on iPhone and iPad must use the same Apple ID to enable this feature