How do I pair my iPhone X to my iPad mini to make phone calls

Follow this article at the cellular calls section

Use Continuity to connect your Mac, iPhone, iPad, iPod touch, and Apple Watch - Apple Support

Sign in to FaceTime on your iPhone and your iPad. Make sure that your phone number is selected for “Send and Receive” in FaceTime settings on the iPad.

You don't pair iPhone with iPad.

1. Use the same Apple ID and password for both devices

2. On iPhone

Settings > Phone > Calls on Other Devices > you iPad should be selected here