There are multiple posts in this forum which point to turning off one or more of the following:
1) VoLTE calling - seems to be an option on locked phones
2) WiFI calling - by default, my unlocked phone was set to use WiFI calling - open up your phone app and go to the Keypad display --> open the menu (vertical ...) in the upper right --> select settings. Under Call settings, there should be an option to turn off Wi-Fi calling.
Hope that helps!
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