Trigger for validating phonenumber

Create a trigger which gets triggered when there is any inserting or updating on phone number column of the customer table and this trigger has to capture the customer number and check that number having 10 digits or not.

create or replace trigger number_trigger
before insert or update on customers
for each row
begin
if length(:old.customer_number)>10 then
raise_application_error(-20500,'length is exceeded');
elseif inserting then 
dbms_output.put_line('number inserting');
elseif updating then
dbms_output.put_line('updating number');
end if;
end;
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