You first set up your iPhone, and then set up your iPad.
On your iPhone, go to Settings > Cellular.
If your iPhone has Dual SIMs, choose a line (below SIMs).
Do any of the following:
Tap Calls on Other Devices, turn on Allow Calls on Other Devices, then choose your iPad along with any other devices on which you’d like to make and receive calls.
This allows iPad and other devices where you’re signed in with the same AppleID to make and receive calls when they’re near your iPhone and connected to Wi-Fi.
Tap Wi-Fi Calling, turn on Wi-Fi Calling on This iPhone, then turn on Add Wi-Fi Calling For Other Devices.
This allows iPad and other devices where you’re signed in with the same AppleID to make and receive calls even when your iPhone isn’t nearby.
On your iPad, go to Settings> FaceTime, then turn on FaceTime and Calls from iPhone. If you’re asked, turn on Wi-Fi Calling.
Make a call: Tap a phone number in Contacts, Calendar, FaceTime, Messages, Search, or Safari. Or open FaceTime, enter a contact or phone number, then tap .
Receive a call: Swipe or tap the notification to answer or ignore the call.
Note: If you enable Wi-Fi Calling, emergency calls may be made over Wi-Fi, and your device’s location information may be used for emergency calls to aid response efforts, regardless of whether you enable Location Services. Some carriers may use the address you registered with the carrier when signing up for Wi-Fi Calling as your location.