create or replace PROCEDURE top_protected_proc
  ACCESSIBLE BY (PROCEDURE top_trusted_proc)
AS
BEGIN
  DBMS_OUTPUT.PUT_LINE('Executed top_protected_proc.');
END;
/
create or replace PROCEDURE top_trusted_proc AS
BEGIN
  DBMS_OUTPUT.PUT_LINE('top_trusted_proc calls top_protected_proc');
  top_protected_proc;
END;
/

SQL> EXEC top_trusted_proc;
top_trusted_proc calls top_protected_proc
Executed top_protected_proc.

PL/SQL procedure successfully completed.

SQL> EXEC top_protected_proc;
BEGIN top_protected_proc; END;

      *
ERROR at line 1:
ORA-06550: line 1, column 7:
PLS-00904: insufficient privilege to access object TOP_PROTECTED_PROC