How to activate Cellular service for iPad Air

Start here:

Set up cellular service on iPad (Wi-Fi + Cellular models) - Apple Support

An iPad, whether a Cellular model or not, is not capable of directly accessing Cellular Voice or SMS/MMS Messaging features. An iPad with either WiFi or Cellular connectivity alone can only access IP-data services.

If you have added an eSIM or a physical DataSIM and Cellular plan to a WiFi+Cellular model of iPad, the number associated with the eSIM/SIM card is only used to identify your account for billing purposes. This number is not used for making calls or sending messages.

However, when paired with an iPhone, the capabilities of iPad significantly broaden to include access to Cellular Voice and Messaging services of the associated iPhone - relayed to the iPad over WiFi using Apple’s continuity features.

More information about Continuity and the relevant Cellular services can be found here:

Continuity

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

SMS/MMS Messaging

How to forward SMS/MMS text messages from your iPhone to your iPad, iPod touch or Mac - Apple Support

WiFi Calling

Make a call with Wi-Fi Calling – Apple Support

Almost. You need a separate connection, which will have an identifying number that probably looks like a phone number (depending on your telecommunications carrier) but cannot be used for phone calls or SMS/MMS messaging.

Assuming the iPad Air you bought included the cellular hardware rather than being the wifi-only model, you may be able to activate cellular service from within the Cellular panel of the Settings app. Again, this may depend on your carrier.

If your Settings app doesn't have a Cellular panel, you may have bought a wifi-only iPad.

iPads don’t have telephone numbers and can’t make phone calls.