Something

BEGIN
    LOCK TABLE city_calls;
    SELECT INTO _res MAX(call_nb)+1 FROM city_calls;

    IF _res IS NULL THEN
        _res := 0;
    END IF;

    INSERT INTO city_calls(unique_id,call_nb) VALUES(_uid,_res);

    IF NOT FOUND THEN
        RETURN -1;
    ELSE
        RETURN _res;
    END IF;
END;