In reading Apples Website it appears to me that there are two different models for each phone although they use the same model number one labeled (CDMA) and one (GSM). I called apple and the customer service rep was not sure of the difference. This leads me to believe that the phones purchased as unlocked or Verizon / Sprint are CDMA and support GSM as well, meanwhile the phones purchased from Tmobile or ATT are GSM Only. There is also an article attached that supports my theory.
https://www.apple.com/iphone/LTE/
"But even though Apple reportedly shifted this year from a mix of Qualcomm and Intel modems to purely Intel parts, there are still separate GSM and CDMA versions of each of the phones. It’s unclear whether one version (historically, the CDMA model) will be a smarter purchase. (And there are even more international models than before, now compounded by separate eSIM and physical SIM models, but that was underscored during the event.)"
https://www.apple.com/iphone/LTE/
- Phone XS
- Model A1920 (GSM)
- Model A1920 (CDMA)
- iPhone XS Max
- Model A1921 (GSM)
- Model A1921 (CDMA)
"But even though Apple reportedly shifted this year from a mix of Qualcomm and Intel modems to purely Intel parts, there are still separate GSM and CDMA versions of each of the phones. It’s unclear whether one version (historically, the CDMA model) will be a smarter purchase. (And there are even more international models than before, now compounded by separate eSIM and physical SIM models, but that was underscored during the event.)"