Why a Cellular plan if you have to use an iPhone for the call?

An iPad, whether a Cellular model or not, is not capable of directly accessing Cellular Voice or Messaging features. An iPad with Cellular connectivity alone can only access IP-data services. An iPad is not, and can never be, a standalone substitute for an iPhone...

When coupled 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. For the most part, an iPad using Continuity is able to access Cellular calling features, of the iPhone, when both the iPad an iPhone are in close physical proximity and are connected to the same WiFi network - but this is not the end of the story.

Now, if your both your Cellular Carrier and your calling plan support WiFi Calling, your iPad may be able to make and receive calls independently of the iPhone - even when the iPhone is switched off, elsewhere, or using a different WiFi network. Not all carriers support full WiFi calling from associated devices; you should note that WiFi calling from iPad, where supported, will still be using the iPhone's mobile phone number (with the Cellular Carrier) even when making/receiving calls independently of the associated iPhone.

Where supported by the iPhone's Cellular plan, the settings that you need to change/verify on your iPhone and iPad are:

On your iPhone

Settings > Mobile Data >WiFi Calling > 

• WiFi Calling on This iPhone - set to ON
• Add WiFi Calling for Other Devices - set to ON (only seen if the Carrier plan supports full WiFi Calling)

Settings > Mobile Data > Calls on Other Devices > 

• Allow Calls on Other Devices - set to ON
• [Allow Calls On] - explicitly enable your new iPad from the list of devices.

On your iPad

Settings > FaceTime > Calls from iPhone > Calls from iPhone - set to ON

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

Continuity

Work across all of your devices seamlessly – Apple Support

SMS/MMS Messaging

Forward SMS/MMS text messages from your iPhone to your Mac or iPad - Apple Support

WiFi Calling

Make a call with Wi-Fi Calling – Apple Support

Making and receiving calls on iPad

Make and receive phone calls on iPad - Apple Support

In conclusion, in conjunction with an iPhone, it is theoretically possible to configure WiFi Calling service for your iPad. After WiFi Calling is enabled and configured, your iPad may be able to make and receive cellular calls whilst connected to a WiFi network even in absence of the iPhone.

WiFi Calling from iPad does not work over a Cellular data connection.

It's a DATA traffic plan, data used by the iPad to access the Internet when your device cannot connect (because not available) to any wifi network.

Regards

Giulio

My pleasure :)