You can allow your users to select a country code for the phone field. This is not shown by default so you have to set it up. You need to go to:
Form Fields
Click the phone field
Advanced section
Turn on the Show Country Codes toggle.
To set a default country code, you must set a value in the Prefill Country Code (Optional) field.
This feature is only available for fields on the form and child forms, not for login fields.