Try a quick SIM swap. Eject the iPad SIM. Put it in any phone. On iPhone, open Settings, Phone, My Number. On Android, Settings, About phone, SIM status, My phone number. If it shows Unknown, send a test text to anothr phone to read the number. Move the SIM back to the iPad. Total time, 2 to 3 minutes. Works on AT&T, T-Mobile, Verizon in the US. For eSIM only, open your carrier app or web acccount on the iPad. The line name shows the number on the plan page.