I can’t remember any one time that someone has gotten this right. They just expect you to know the format to input. This really sucks when someone’s new to a country and not really well versed on how phone numbers in a country look like.
So please, don’t disregard the design portion of this input field. The easier your form is to understand the quicker users will finish it. Which, I’m guessing, is what you want, no?
Oh and Alternative 1 is the best option for me. You could also make it so that the suggestion is inside the field (using