It’s not straight forward.
A cellular iPad doesn’t have a (native) phone app, so you can’t just dial a number like you would with an iPhone.
Nor can it (natively) receive inbound calls (the phone number attached to the SIM card is buried away).
There are some 3rd party apps that emulate a phone (I think) but they are VoIP, so not ‘real‘ phone calls. Texting (SMS) also a PITA and requires a 3rd party app which still doesn’t use the cellular modem directly.
You’d have to say that Apple have deliberately stymied the use of an iPad as a mobile phone. I’m not sure why.
Anyway, the other apps mentioned above work OK on an iPad: Skype, Google Duo, Hangouts and FaceTime etc. BUT NOT WhatsApp (and WhatsApp video calling).
Maybe better off with a phone or the Galaxy Note. That does do it all. But it’s pricey (although Boxing Day sales give it £150 off) and you can buy on a monthly contract:
Samsung Galaxy Note 10
Example contract – Skymobile (not much data)