The phone field at checkout shows a country dropdown, and you may want it to start on a country other than the US.
How the default is set. The checkout widget has a setting called Top Countries. It accepts a comma-separated list of two-letter country codes. These codes appear at the top of the phone country dropdown, and the first code in the list becomes the default. When Top Countries is left empty, the checkout defaults to the US (+1).
Changing it. Enter your preferred country code first in Top Countries. For example, to default to the UK, set it to "gb,us" or just "gb". The change takes effect on the live checkout within a few minutes and does not require republishing your site.