Herewith a full explanation...
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.
However, 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. To do so, both devices must be signed-in to iCloud with the same AppleID.
For the most part, an iPad using Continuity is able to access Cellular calling features of the associated iPhone when both the iPad an iPhone are in close physical proximity and are connected to the same WiFi network.
If your both your Cellular Carrier and your calling plan support WiFi Calling, the iPad can 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.
Remember, both devices must be using the same AppleID. Settings that you may need to change/verify on 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
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
When correctly configured, the iPad FaceTime App provides access to Cellular Voice Calling features - whilst Messages supports SMS/MMS messaging services.
If you simply wish to use the Cellular hotspot feature of your iPhone with other devices, including Apple devices that are using a different AppleID, you'll need to manually enable the iPhone hotspot:
Settings > Personal Hotspot > Allow Others to Join - set to ON
NB: Make note of the Wi-Fi Password - as this will be needed by other devices when connecting to the hotspot.
Once enabled, you can simply join the hotspot as you would any WiFi network from device settings.