Check your SIM per below. Go to settings/about and check the IMEI number
SIM card compatibility
AT&T HD Voice requires a SIM card that is UICC-G or higher SIM card installed in your smartphone to enjoy the rich calling experience of AT&T HD Voice. Want to make sure to find out if your SIM card is HD Voice compatible? In the SIM card number, look for the digits
27 or higher in the 9th and 10th positions.
Enjoy crystal-clear conversations and reduced background noise with AT&T HD Voice on our network.
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