An iPad, whether a Cellular model or not, is not capable of directly accessing Cellular Voice or SMS/MMS Messaging features. An iPad with either WiFi or Cellular connectivity alone can only access IP-data services. If you have added an eSIM or physical DataSIM and Cellular plan to a WiFi+Cellular model of iPad, the number associated with the eSIM/SIM card is only used to identify your account for billing purposes. This number is not used for making calls or sending messages; an iPad is not, and can never be, a standalone substitute for an iPhone.
However, when combined 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.
More information about Continuity and the relevant Cellular services can be found here:
Continuity
Use Continuity to connect your Mac, iPhone, iPad, iPod touch and Apple Watch – Apple Support
SMS/MMS Messaging
How to forward SMS/MMS text messages from your iPhone to your iPad, iPod touch or Mac - Apple Support
WiFi Calling
Make a call with Wi-Fi Calling – Apple Support