iPad calls

Whether your iPad is cellular or not, it is NOT a cell phone and it does not have a cellular radio. Cellular service on an iPad is for Data, when you cannot use Wifi.

You can use Continuity Services to make a receive calls using an iPad IF and only IF you have an iPhone. If you don't have an iPhone, you will not be able to make or receive calls. But you could use a service, such as Skype.

If you have an iPhone see this link about using Continuity Services --> Use Continuity to connect your Mac, iPhone, iPad, iPod touch, and Apple Watch - Apple Support

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, If 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. 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 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 - and many Carrier representatives don’t technically understand or acknowledge its existence.

[NB: Many contributors here don’t understand the full implementation of WiFi Calling either - perhaps because this feature is often only available from some full-service cellular carriers; cut-price MVNO cellular operators may not offer the service at all].

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

Continuity

https://support.apple.com/en-gb/HT204681

SMS/MMS Messaging

https://support.apple.com/en-us/HT208386

WiFi Calling

https://support.apple.com/en-gb/HT203032

When correctly configured, the iPad FaceTime App provides access to Cellular Voice Calling features - whilst Messages supports SMS/MMS messaging services.

So, in conclusion, if your own Cellular Carrier and tariff support WiFi Calling, it is theoretically possible to configure WiFi Calling service for your iPad - before switching off and removing the SIM card from an iPhone (e.g., before sending for repair). After WiFi Calling is enabled and configured, your iPad can continue to make and receive cellular calls whilst connected to a WiFi network (caveat: any associated firewall must not block WiFi Calling or VoIP services) - even in absence of the iPhone.

I hope this information and clarification is helpful in resolving your immediate needs.