This attribute contains the user data, if present. userData is a 0 to 16
length string which is appended to the call request packet that is
presented to the called (destination) DTE.
User data can be a 0 to 128 byte string for fast select calls;
otherwise it is 0 to 16 byte string.
Fast select calls are indicated as such using the X.25 ITU-T facility
for 'Reverse Charging'.
The length of the user data attribute is not verified during service
provisioning. If more than 16 bytes of user data is specified on a
call without the fast select option, then the call is cleared with a
clear cause of 'local procedure error', and a diagnostic code of 39
(as defined in the ITU-T X.25 Recommendation).