An iPad, whether a Cellular model or not, is not capable of accessing Cellular Voice or Messaging features. An iPad with Cellular connectivity alone can only access IP-data services. An iPad is not an oversized iPhone.
If paired with an iPhone, the capabilities of iPad significantly broaden to include Cellular Voice and Messaging services of the iPhone - relayed to the iPad over WiFi using Apple’s continuity features.
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
To make and receive voice calls from iPad, over Cellular, your iPad must be linked with a iPhone. Calls are made using the FaceTime App; there isn’t a “Phone” App for iPad.
Other third-party calling Apps, such as WhatsApp, make calls over IP-data (often using VoIP); they do not make Cellular Voice calls over Cellular networks.
I hope this information and clarification is helpful in resolving any issues you may have.