Tethering an iPhone to an iPad
can I tether my iPhone to my iPad such that it acts like a phone?
iPad Pro 12.9-inch, 2nd Gen, Wi-Fi
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Your iPhone can provide SMS/MMS services to your iPad - and if your iPhone’s Cellular provider supports WiFi Calling on their network, your iPad can make voice calls via your iPhone. Voice calls are received and initiated from your iPad using the FaceTime App.
https://support.apple.com/en-gb/HT208386
https://support.apple.com/en-gb/HT209456
You should note that you iPad cannot access Cellular service features directly - but instead is reliant upon Cellular features of the iPhone. Apple’s continuity feature-set provides the link between the two devices.
I hope this information is helpful.