Originally Posted by
König
....but when you are using a foreign SIM card and want to call a 10-digit US number from Hangouts dialer, you have to prepend it with +1 (e.g., +1 234-567-8910).
As you do when dialing any foreign number from any phone, otherwise it won't know which country you are trying to call. That's why it's always sensible to ensure you have the country code in all your contacts.