phnumtest2.cs

    [TestClass]
    public class UnitTest1
    {
        [TestMethod]
        public void TestMethod1()
        {
            String s1 = TAMSLib.PhoneNumber.FormatUSA("5031234567");
            String s2 = TAMSLib.PhoneNumber.FormatUSA("1(503)1234567");
            String s3 = TAMSLib.PhoneNumber.FormatUSA("5031234567x346354");
            String s4 = TAMSLib.PhoneNumber.FormatUSA("1(503)1234567x63456");
        }
    }

    output:
    (503) 123-4567
    +1 (503) 123-4567
    (503) 123-4567 x346354
    +1 (503) 123-4567 x63456

    public class PhoneNumber
    {
        static public string FormatUSA(string input)
        {
            StringBuilder sb = new StringBuilder();
            StringBuilder phone = new StringBuilder();
            StringBuilder extension = new StringBuilder();
            string rv = "";
            bool bHasExtension = false;

            if (input.ToUpper().IndexOf("X") > 0)
            {
                var ext = input.ToUpper().IndexOf("X");
                foreach(var c in input)
                {
                    if(c.ToString().ToUpper().Equals("X"))
                    {
                        bHasExtension = true;
                        continue;
                    }
                    if (!bHasExtension)
                    {
                        phone.Append(c);
                    }
                    else
                    {
                        extension.Append(c);
                    }
                }
            }
            else
            {
                phone.Append(input);
            }

            foreach (var c in phone.ToString())
            {
                if ("0123456789".IndexOf(c) >= 0)
                {
                    sb.Append(c);
                }
            }
            if (sb.Length == 10)
            {
                rv = string.Format("({0}{1}{2}) {3}{4}{5}-{6}{7}{8}{9}", sb[0],sb[1],sb[2],sb[3],sb[4],sb[5],sb[6],sb[7],sb[8],sb[9]);
                if (bHasExtension)
                {
                    rv = string.Format("({0}{1}{2}) {3}{4}{5}-{6}{7}{8}{9} x{10}", sb[0], sb[1], sb[2], sb[3], sb[4], sb[5], sb[6], sb[7], sb[8], sb[9], extension);
                }
            }
            else if (sb.Length == 11)
            {
                rv = string.Format("+{0} ({1}{2}{3}) {4}{5}{6}-{7}{8}{9}{10}", sb[0], sb[1], sb[2], sb[3], sb[4], sb[5], sb[6], sb[7], sb[8], sb[9], sb[10]);
                if (bHasExtension)
                {
                    rv = string.Format("+{0} ({1}{2}{3}) {4}{5}{6}-{7}{8}{9}{10} x{11}", sb[0], sb[1], sb[2], sb[3], sb[4], sb[5], sb[6], sb[7], sb[8], sb[9], sb[10], extension);
                }
            }
            else
            {
                rv = input;
            }

            return rv;
        }
    }