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Simply move your Cellular SIM card from your iPad Air to your iPad mini:

Remove or switch the SIM card in your iPad - Apple Support

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.

Take the SIM out of the old iPad and put it into the new one.

Note that iPads have "cellular numbers" assigned to them for billing purposes only. They cannot be used for calling or texting.