I have an iPhone 6s+ on AT&T (still under contract) and an iPad Air 2 with an AT&T Sim. iPad plan was pre-paid and had expired - so I had no active cell service on the iPad. After reading about the T-Mobile $10/5 Gb iPad plan, I got a T-Mobile sim and tried to sign up. Kept getting errors. Called customer support and they had me try to sign up both on the iPad and via Safari on my iMac. They finally got it to almost work, but their system kept sending a validation code via text message to my iPad (to the new number assigned by T-Mobile) that I needed to type in to complete the sign up. However, iMessage on my iPad will not allow me to send or receive messages with that number. It only allows me to use the AT&T number from my iPhone. I tried disabling "allow calls on other devices" from the iPhone but still no success. Tried changing the number on the iPad via Settings->Messages but no luck. I finally had to borrow an unlocked iPhone and insert the T-Mobile sim to receive the text. My iPad is now on T-Mobile and working great but I still can't receive texts from the T-Mobile number. The CSR said T-Mobile will occasionally send things to me via text in the future, but I don't see how I will be able to receive these texts. Is it even supposed to use that number? When I had the iPad on AT&T, it never used the number AT&T assigned to it, instead it always used the iPhone number. Any ideas?