Converting a normalized phone number to a user-friendly version

String.Format("{0:(###) ###-#### x ###}", double.Parse("1234567890123"))

(d{3})(d{3})(d{4})(d{4})

(1) 2-3 x4

class Program
{
    static void Main(string[] args)
    {
        long phoneNumber = 12345678901234;
        string phoneNumberString = String.Format("{0:(000) 000-0000 x0000}", phoneNumber);
        Console.WriteLine(phoneNumberString);
    }
}

public static string InsertStringAtPositions(this string str, string insertStr, IEnumerable<int> positions)
{
    if (str != null && insertStr != null && positions != null)
    {
        string newString = string.Empty;
        int previousPos = 0;
        foreach (var pos in positions)
        {
            if (pos < str.Length)
            {
                newString += str.Substring(previousPos, pos - previousPos) + insertStr;
                previousPos = pos;
            }
        }
        if (positions.Last() < str.Length)
        {
            return newString + str.Substring(positions.Last(), str.Length - positions.Last());
        }
        return newString;
    }
    return str;
}

// Will convert "0399998888" to "03 9999 8888"
number.InsertStringAtPositions(" ", new[] {2, 6});