Originally Posted by
lhrsfo
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.
You are absolutely right, but it may not be so obvious to infrequent American travellers whose contact numbers are almost always stored in 10- or 11-digit format without a leading +. I just wish that Hangouts app could spew a bit more useful message than their generic "invalid format" one.