Suppose Table T stores teh phone nos..(in column tel_no which is varchar2)

then to do this add a check constraint like:

  1* ALTER TABLE T ADD (CONSTRAINT T_C CHECK (TO_NUMBER(REPLACE(TEL_NO,'-'))>0))
SQL> /

Table altered.

--Now try adding something invalid like:

SQL> INSERT INTO T VALUES('ASD-343');
INSERT INTO T VALUES('ASD-343')
          *
ERROR at line 1:
ORA-01722: invalid number --constraint throws out the error..