Specifically, will the $70 unlimited T-Mobile PHONE sim work in a Verizon or AT&T iPad Air? I know many still have their grandfathered AT&T or Verizon phone plan. What about those who have the unlimited T-Mobile?
I reckon both AT&T and T-Mobile both know what Apple device you have based on IMEI and may be blocking subscribers from using phone plans on tablets. I wouldn't chance it. Try if it works on a cheaper phone plan first before you switch to the $70 plan.