C#设置电话只能输入数字

string sValue = this.txtFax.Text.ToString().Trim();
            char[] sChar = sValue.ToCharArray();
            foreach (char str in sChar)
            {
                if (!char.IsNumber(str))
                {
                    Msg.InfoBox("联系电话只能输入数字！");
                    return false;
                }
            }