While adding a Phone Number field in forms, you will see an option called "Use Country Codes Instead of Flags":
Why and when should you enable this option?
By default, (i.e. when the "Use Country Code.." option is not enabled) the phone number dropdown will show flags only:
However, some browsers OR some website hosts/plugins may not support flag rendering. To avoid this, you should use country codes instead of flags.
This is what your phone number dropdown will look like when this option is enabled:
As you can see above, the dropdown will show the respective country codes instead of flags.