How to format a phone numbers with libphonenumber in International format.

Have you tried this?

var phoneUtil = PhoneNumberUtil.GetInstance();
var numberProto = phoneUtil.Parse("1234567890", "IT");
var formattedPhone = phoneUtil.Format(numberProto, PhoneNumberFormat.INTERNATIONAL);

This will give you "+39 123 456 7890"

@danyolgiax almost had it. Here's what you do:

var phoneUtil = PhoneNumberUtil.GetInstance();
var numberProto = phoneUtil.Parse("+4915213116250", "ZZ"); //why ZZ means international i don't know
var formattedPhone = phoneUtil.Format(numberProto, PhoneNumberFormat.INTERNATIONAL);

Remember to use phoneUtil.IsValidNumber if you aren't certain that the phone number is valid. Also be aware that phoneUtil.Parse may throw.