Prettify phone numbers
If you don't want to handle an error, then ignore it by using the _ symbol.
I.E. replace:

file, err := os.Open("./phonenumbers.txt")
check(err)


with:

file, _ := os.Open("./phonenumbers.txt")

Move the entire scanning logic into a separate function (you did write you are fond of functional programming ).
func PrettifyNumber(original string) string
You could use the strings.Fields to filter out any non numeric character.  

func IsNotADigit(r rune) bool {
    return !unicode.IsDigit(r)
}

func PrettifyNumber(original string) string {
    return strings.Join(strings.FieldsFunc(original, IsNotADigit), " ")
}