MaskMoney.cs

public class MaskMoney : Behavior<Entry>
    {
        protected override void OnAttachedTo(Entry bindable)
        {
            bindable.TextChanged += onTextChanged;

            base.OnAttachedTo(bindable);
        }

        protected override void OnDetachingFrom(Entry bindable)
        {
            bindable.TextChanged -= onTextChanged;

            base.OnDetachingFrom(bindable);
        }

        void onTextChanged(object sender, TextChangedEventArgs args)
        {
            var entry = (Entry)sender;

            entry.Text = formatPhoneNumber(entry.Text);
        }

        private string formatPhoneNumber(string input)
        {
            //var digitsRegex = new Regex(@"[^\d]");
            var digits = input.Replace(input, "");

            if (digits.Length < 3)
                return "0," + input;
                //return $"0,{input}";

            if (digits.Length >= 3 && input.Length <= 5)
                return digits.Substring(2, 3) + "," + digits.Substring(0, 2);
                //return $"{digits.Substring(2, 3)},{digits.Substring(0,2)}";

                //return $"{digits.Substring(4)}.{digits.Substring(2, 3)},{digits.Substring(0, 2)}";
            return digits.Substring(4)+"."+ digits.Substring(2, 3) + ","+digits.Substring(0, 2);
        }
    }