RadmaskedEditBox should not allow you to type hyphens (or other special characters).

        public RadForm1()
        {
            InitializeComponent();

            this.radMaskedEditBox1.MaskType = MaskType.Standard;
            this.radMaskedEditBox1.Mask ="(###) ###-#### & & & & & & & &";
            this.radMaskedEditBox1.MaskedEditBoxElement.Provider = new CustomStandartMaskTextBoxProvider(this.radMaskedEditBox1.Mask,
                this.radMaskedEditBox1.Culture, 
                this.radMaskedEditBox1.MaskedEditBoxElement, false, '_', '*', true);  
        }
         

        public class CustomStandartMaskTextBoxProvider : StandartMaskTextBoxProvider
        { 
            public CustomStandartMaskTextBoxProvider(string mask, CultureInfo culture, RadMaskedEditBoxElement owner,
                bool allowPromptAsInput, char promptChar, char passwordChar, bool restrictToAscii) : 
                base(mask, culture, owner, allowPromptAsInput, promptChar, passwordChar, restrictToAscii)
            {
            }
            public override void KeyPress(object sender, KeyPressEventArgs e)
            {
                if (e.KeyChar=='-')
                {
                    e.Handled = true;
                    return;
                }
                base.KeyPress(sender, e);
            } 
        }