If you need to call and your iPhone is not nearby, you can use your iPad or Mac instead thanks to a feature known as iPhone Cellular Calls. It's part of Apple's Continuity system, designed to sync and share some features over iOS, Apple Watch and Mac, and also includes Handoff and Universal Clipboard.
As long as your iPhone is not too far away, and you set up both your phone and your tablet with the right options, your iPad can fill in for phone calls. And if your iPad is not nearby but you are in front of your Mac, you can make and receive calls that way too. Let's see how all this plays out.