SQL & PL/SQL :: Using Dbms_lock In Schema?

When I log into 'SYSTEM' schema I am able to use the dbms_lock command as followed:

declare
begin
dbms_lock.sleep(3);
end;

But i need to call it from my own 'INT' schema

declare
begin
system.dbms_lock.sleep(5);
end;

ORA-06550: line 3, column 5:
PLS-00201: identifier 'SYSTEM.DBMS_LOCK' must be declared
ORA-06550: line 3, column 5:
PL/SQL: Statement ignored

And also from the 'SYSTEM' schema i tried to grant privledges to public

grant execute on dbms_lock to PUBLIC;

but that gives me ORA-01031: insufficient privileges