While you can lend a suitably prepared iPad to another user, as described by sberman, you might not achieve your suggested aim. An iPad is not, and can never be, a standalone substitute for an iPhone.
An iPad, whether a Cellular model or not, is not capable of directly accessing Cellular Voice or Messaging features. An iPad 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 SIM card is only used to identify the account for billing purposes. This number is not used for making calls or sending messages.
When coupled with an active iPhone, the capabilities of iPad 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.