PhoneTextBox

    public class PhoneTextBox : TextBox
    {
        public string CountryCode
        {
            get { return (string)GetValue(CountryCodeProperty); }
            set { SetValue(CountryCodeProperty, value); }
        }

        public static readonly DependencyProperty CountryCodeProperty =
            DependencyProperty.Register("CountryCode", typeof(string), typeof(PhoneTextBox), new PropertyMetadata(null, OnCountryCodeChanged));

        private static void OnCountryCodeChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            var sender = (PhoneTextBox)d;
            var newValue = (string)e.NewValue;
            var oldValue = (string)e.OldValue;

            if (newValue != oldValue)
            {
                sender.Formatter = PhoneNumberUtil.GetInstance().GetAsYouTypeFormatter(newValue);
                sender.Text = sender.GetFormatted();
                sender.SelectionStart = sender.Text.Length;
            }
        }

        private AsYouTypeFormatter Formatter;

        protected override void OnKeyDown(KeyRoutedEventArgs e)
        {
            if (Formatter != null)
            {
                if (IsValidKey(e.Key))
                {
                    Text = Text.Insert(SelectionStart, ValueFromKey(e.Key));
                    Text = GetFormatted();
                    SelectionStart = Text.Length;
                }
                else if (e.Key == VirtualKey.Back)
                {
                    if (SelectionStart > 0)
                    {
                        Text = Text.Remove(SelectionStart - 1, 1);
                        Text = GetFormatted();
                        SelectionStart = Text.Length;
                    }
                }
            }

            e.Handled = true;

            base.OnKeyDown(e);
        }

        private string GetFormatted()
        {
            Formatter.Clear();

            var text = Regex.Replace(Text, "[^0-9]", "");
            var result = string.Empty;
            for (int i = 0; i < text.Length; i++)
            {
                result = Formatter.InputDigit(text[i]);
            }

            return result;
        }

        private bool IsValidKey(VirtualKey key)
        {
            var name = Enum.GetName(typeof(VirtualKey), key);
            if (name.StartsWith("Number"))
            {
                return true;
            }

            return false;
        }

        private string ValueFromKey(VirtualKey key)
        {
            var name = Enum.GetName(typeof(VirtualKey), key);
            var value = Regex.Match(name, "([0-9])");

            return value.Value;
        }
    }