iPhone 8 not compatible

[Cpacella4]

I brought 3 iPhones over from my AT&T account. The iPhone 12 and iPhone 10 are working well although the iPhone 12 intermittently says no SIM card (very annoying). The iPhone 8 was working but now it cannot make or receive calls because it is not compatible. If it's not compatible why was it working when we first set up the account but now it is not???

[wpaulo]

The modem in the iPhone 8 was manufactured with both Qualcomm and Intel chips. Most sold or for ATT used Intel which are not compatible. You might have been lucky/unlucky and got an LTE channel that connected.

[DaveO3]

iPhone 8 and 8 Plus had non CDMA  verisions that could not be used on  Verizon and therefore XM. 

• Model A1905*
  Models A1905 and A1897 do not support CDMA networks, such as those used by Verizon and Sprint.
  FDD-LTE (Bands 1, 2, 3, 4, 5, 7, 8, 12, 13, 17, 18, 19, 20, 25, 26, 28, 29, 30, 66)
  TD-LTE (Bands 34, 38, 39, 40, 41)
  UMTS/HSPA+/DC-HSDPA (850, 900, 1700/2100, 1900, 2100 MHz)
  GSM/EDGE (850, 900, 1800, 1900 MHz)

HOWEVER - The good news is that CDMA 2G/3G networks have mostly been deactivated as of January 1st, 2021. But, some of the old CDMA provisioning and white listing still remains as the switch has been made to LTE and voLTE. 

THE FIX - Verizon customers can have their phones provisioned to be CDMA-less. I don't know whether XM is doing this yet. Look up CDMA-less for more information. 

Good luck and let us know if you are successful.