As noted, it looks like you're calling beautifyPhoneNumber on a number you've already beautified.
Here's an implementation using regular expressions that should get you started:
public static String BeautifyPhoneNumber(string numberToBeautify)
{
//The below gives us capture groups for each
//individual piece of the number.
var regularExpression = new Regex(@"(\d{3})(\d{3})(\d{4})(x\d*)?");
//This matches a number that's already been beautified,
//so we can guard against beautifying twice.
var alreadyBeautifulExpression = new Regex(@"(\(\d{3}\)) (\d{3})-(\d{4}) ?(x\d*)?");
var beautifulNumber = string.Empty;
var separator = "-";
var space = " ";
//This prevents us from accidentally beautifying
//something more than once
//You could also guard against this in your getter using a
//IsBeautified extension, using the alreadyBeautifulExpression above
if (alreadyBeautifulExpression.IsMatch(numberToBeautify))
{
return numberToBeautify;
}
//Trying to protect against invalid input... May be insufficient,
//Or unnecessary
if (string.IsNullOrEmpty(numberToBeautify)
|| regularExpression.Matches(numberToBeautify).Count <= 0)
{
return beautifulNumber;
}
GroupCollection groups = regularExpression.Matches(
numberToBeautify)[0].Groups;
//More protection against invalid input
if (groups.Count < 3)
{
return beautifulNumber;
}
//Given "7689131234",
beautifulNumber += "(" + groups[1] + ")" + space; //gives us "(768) "
beautifulNumber += groups[2] + separator; //gives us "(768) 913-"
beautifulNumber += groups[3]; //gives us "(768) 913-1234"
//If we have an extension, we add it.
if (groups[4] != null)
{
beautifulNumber += space + groups[4];
}
return beautifulNumber;
}
Given inputs of:
7279884545
7279884545x12
(727) 988-4545
This returns:
(727) 988-4545
(727) 988-4545 x12
(727) 988-4545