Select a default country code for phone numbers

1. Open Regional Settings.
2. Under Phone Numbers, tap Region > tap a country.
3. Tap Apply Changes and Restart > Apply and Restart. To save the change without restarting, tap Apply Only. The change doesn't take effect everywhere until your device restarts.

This region is for contacts and phone numbers that don't carry a country code. The Phone app currently uses the region under Formats instead (learn how).