returns year as 4 instead of 2004! and phone number

This returns a single number 4, instead of 2004!
select to_number(to_char(to_date(enrolment_date, 'DD-MM-YYYY'), 'YYYY')) into Y from students;


and when i insert a phone number it is something like "020..." but the sql trims off the leading 0, how can i avoid it?

Change the Query as
select to_number(to_char(to_date(enrolment_date, 'DD-MM-YY'), 'YYYY')) into Y from students;

This should work for you.
For inserting Phone no. Keep your data type for this column as either Char or varchar(2).

thanks