Cellular data number unknown on iPad
Cellular data number unknown on iPad, how can I fix it?
The Cellular number associated with your iPad’s Data-SIM, if installed, is only used for account identification and billing purposes. The Cellular number associated with the Data-SIM, if present, serves no other function.
Unless you also have an iPhone, your iPad cannot send or receive SMS or MMS messages to phone numbers - as these are Cellular Carrier features. Without an iPhone, your iPad is limited to sending/receiving messages via the Apple-proprietary Messages App.
Native iMessages are directed to Apple recipients by their registered email addresses - and not by their Cellular phone numbers.