Adding additional information to that already provided by Rudegar...
Your iPad doesn’t natively provide any Cellular services - and doesn’t have a phone number. Your iPad may use its Continuity features to send/receive SMS/MMS messaging, or use Cellular WiFi-Calling services provided by your iPhone if you have one.
If you have a Cellular model of iPad, with a Cellular Data plan, the “number” is simply used for billing purposes. As such, it can only be changed by changing your Cellular Service Plan or Cellular provider.
Your iPhone can provide SMS/MMS services to your iPad - and if your iPhone’s Cellular provider supports WiFi Calling on their network, your iPad can make voice calls via your iPhone. Voice calls are received and initiated from your iPad using the FaceTime App.
https://support.apple.com/en-gb/HT208386
https://support.apple.com/en-gb/HT209456
I hope this information is helpful.