SQL> CREATE OR REPLACE PACKAGE test_pkg
  2  AS
  3    FUNCTION format_phone
  4      (p_phone IN VARCHAR2)
  5      RETURN      VARCHAR2;
  6    PRAGMA RESTRICT_REFERENCES (format_phone, 'WNDS', 'WNPS', 'RNDS');
  7  END test_pkg;
  8  /

Package created.

SQL> SHOW ERRORS
No errors.
SQL> CREATE OR REPLACE PACKAGE BODY test_pkg
  2  AS
  3    FUNCTION format_phone
  4      (p_phone IN VARCHAR2)
  5      RETURN      VARCHAR2
  6    IS
  7      v_phone     VARCHAR2 (14);
  8    BEGIN
  9      v_phone := '('
 10              || SUBSTR (p_phone, 1, 3)
 11              || ') '
 12              || SUBSTR (p_phone, 4, 3)
 13              || '-'
 14              || SUBSTR (p_phone, 7);
 15      RETURN v_phone;
 16    END format_phone;
 17  END test_pkg;
 18  /

Package body created.

SQL> SHOW ERRORS
No errors.
SQL> SELECT test_pkg.format_phone ('1234567890') AS formatted_phone FROM DUAL
  2  /

FORMATTED_PHONE
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(123) 456-7890