You can use smart dialing to set a default area code and country code that's
applied automatically to the contacts that you call, so you don't have to include a
country code or area code with phone numbers that you add to the Contacts app.
In the Phone app, swipe down
from the top of the screen.
Tap > Smart
Do any of the following:
To change the default country code, tap
Country Code. Tap a country.
To change the default area code, in the Area Code field, type an area code.
Tap any of the drop-down lists to change other smart dialing