SQL & PL/SQL

hello , i need your help .

i would like to know how i can call a procedure that exist in another schema

what i did for example calling the procedure "proc" at schema "user1" from pl/sql code

begin
user1.proc(parm1,parm2);
end;

i got the error message

(1): PLS-00201: identifier 'user1.proc' must be declared
(2): PL/SQL: Statement ignored

but by calling it from SQL PLUS from the same current schema i got no errors

exec user1.proc(parm1,parm2);

thank you