SQL> select banner from v$version;

BANNER
----------------------------------------------------------------
Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - 64bi
PL/SQL Release 10.2.0.4.0 - Production
CORE    10.2.0.4.0      Production
TNS for Solaris: Version 10.2.0.4.0 - Production
NLSRTL Version 10.2.0.4.0 - Production

SQL> create table tmp_table
  2    as select '5607xxx' a_varachar2_col from dual;

Table created.

SQL> select to_number(a_varachar2_col) from tmp_table where a_varachar2_col = '5607xxx';
select to_number(a_varachar2_col) from tmp_table where a_varachar2_col = '5607xxx'
       *
ERROR at line 1:
ORA-01722: invalid number


SQL> 

SQL> alter session set nls_numeric_characters='xy';

Session altered.

SQL> select to_number('5607x') from dual;

TO_NUMBER('5607X')
------------------
              5607

SQL> select to_number('5607xx') from dual;
select to_number('5607xx') from dual
                 *
ERROR at line 1:
ORA-01722: invalid number


SQL> 

alter session set nls_numeric_characters=', ';
select to_number('560,7') from dual; -- 560,7
select to_number('560.7') from dual; -- ORA-01722: invalid number

alter session set nls_numeric_characters='. ';
select to_number('560,7') from dual; -- ORA-01722: invalid number
select to_number('560.7') from dual; -- 560.7