If you need to verify phone number or other fields, and you want to ensure that not all characters can be used, the best is to use a check-constraint. The following constraint is simple, and fast 100 times faster than using a pl/sql-function (tested with 2mio records) because it is used directly on the table and there is no switching between the sql and pl/sql-engine.

alter table customer  add constraint ck_phone_numeric  check ((RPAD(TRANSLATE(phoneno,'0123456789+-X','XXXXXXXXXXXXa'),20,'X')='XXXXXXXXXXXXXXXXXXXX'));

You have to replace the length of the column (in this case 20) to the length of your column and of course the table name...