Hi. Yes you do need an iPhone to make calls. As long as you have an active cellular plan, you can get on the internet without using wifi.
According to https://support.apple.com/guide/ipad/make-and-receive-phone-calls-ipadf97892b2/ipados;
"You can make and receive calls on your iPad by using Wi-Fi Calling to relay phone calls through your iPhone (iOS 9 or later required).
Important: Wi-Fi + Cellular models do not support cellular phone service—they support cellular data transmission only. To make phone calls on any iPad model, you need to use Wi-Fi Calling and an iPhone."
I hope this helps!
One Apple User To Another