You are quite correct in that most Carrier Phone SIMs are not intended to be used with a tablet computer - such as iPad. Your SIM card (and your Cellular plan) must be “provisioned” for explicit cellular services.
An iPad, whether a Cellular model or not, is not capable of directly accessing Cellular Voice or Messaging features. An iPad with Cellular connectivity can only access IP-data services. An iPad is not, and can never be, a standalone substitute for an iPhone.
If you have added a DataSIM and Cellular plan to a WiFi+Cellular model of iPad, the number associated with the SIM card is only used to identify your account for billing purposes. This number is not used for making calls or sending messages.
Carrier plans for tablet computers tend to be considerably more expensive than mobile phone plans because of data usage. IP Data, derived from computers, consume considerable data bandwidth. This differentiation makes cellular data considerably more expensive than “voice” or “messaging” resources.
I hope this information and clarification is helpful.