I’m wondering what value you assigned to the OPT-ACCESS parameter before calling USR1058N? It looks like only “O”, “R”, and “C” are valid, as explained in the USR1058T description below:
Input ...... OPT-ACCESS (A01)
Access type. An open call starts the processing
and returns the directory data. The next calls
read the object until its end. The close call
is done automatically by the subprogram.
Possible values are:
'O' - Open processing
'R' - Read source lines
'C' - Close processing; only necessary in
a file system environment to force the end
Leaving it blank or some other invalid value might result in the 999 Function is not implemented message you are getting.
Hope that helps. If not, it would help if you could post the code you are using to call USR1058N and maybe somebody will spot what you are missing.