When can one not use iPad yo call and receive calls

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.

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

An iPad is not an iPhone - but using the Cellular Carrier features of the associated iPhone airtime contract, is capable of using some of your iPhones calling features.

Both iPad and Phone simply need to be using the same WiFi network - and be signed-in to iCloud with the same Apple AppleID.

If your chosen Cellular Carrier also supports WiFi calling, if configured to do so, iPad can make calls independently of iPhone (via the iPhone’s Cellular Carrier) over any WiFi network.

I hope this clarification is helpful in resolving your query.

They both need to be using the same iCloud account and be on the same WiFi intranet. iOS continuity features only work when the iPhone and other device(s) are on the same local WiFi network.