Be aware that the majority of Phone SIMs are not correctly provisioned for most tablet computers - and most will offer no service at all with an iPad. In most cases, you will require a DataSIM that is explicitly intended for tablet computers and Cellular Routers (also known as MiFi).
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.
The built-in eSIM can also be used to provision cellular service directly with some Cellular Carriers - or by selecting one of the pre-available pre-paid carriers that you can select from the menu.
Set up cellular data service on your Wi-Fi + Cellular model iPad - Apple Support