C# - 18 Digit Unique ID - Code Reliability?

I want a number that would be unique forever, I came up with the following code,
it generates a number and adds a check digit to the end of it, I would like to know how reliable is this code?

public void GenerateUniqueNumber(out string ValidUniqueNumber) {
string GeneratedUniqueNumber = "";
// Default implementation of UNIX time of the current UTC time
TimeSpan ts = DateTime.UtcNow - new DateTime(1970, 1, 1, 0, 0, 0, 0);
string FormatedDateTime = Convert.ToInt64(ts.TotalSeconds).ToString();
string ssUniqueId = DateTime.UtcNow.ToString("fffffff");.......

EDIT: clarification GUID can not be used, the number will need to be entered into a IVR system via telephone keypad.