Getting Top-N Query To Work As Sub-select In Larger Query?

Is there a technique to getting a Top-N query to work as a sub-select in a larger query -or- is there another way to generate Top-N like results that works as a sub-select?

Background:

We have a large query that is being used to build an export from a legacy HR system to a new one. Amount the data needed in the export is the employees primary phone number.

The legacy HR system allows multiple phone numbers to be stored in a simple table structure:

SELECT emp_id, phone_type, phone_number
FROM employee_phones

emp_idphone_typephone_number
------- --------------- -------------------
46021CELL2222222222
46021HOME1111111111
46021WORK3333333333

The new HR system does allow for multiple phone numbers, however they need a primary phone number identified and stored with the employee master information. (Subsequent phone numbers get stored in alternate table.)

From a business perspective, we have decided that if they have a HOME phone in the legacy system that should be the primary in the new system, if no HOME phone, then WORK, if no WORK then CELL.

That can be represented as:

SELECT *
FROM employee_people_phones
WHERE emp_id = '46021'
ORDER BY decode(phone_type, 'HOME', 'a', 'WORK', 'b', 'CELL', 'c', 'z')

emp_idphone_typephone_number
------- --------------- -------------------
46021HOME1111111111
46021WORK2222222222
46021CELL3333333333

Or similarly with Top N concept:

SELECT *
FROM (SELECT *
FROM employee_people_phones
WHERE emp_id = '46021'
ORDER BY decode(phone_type, 'HOME', 'a', 'WORK', 'b', 'CELL', 'c', 'z')) results
WHERE ROWNUM = 1

emp_idphone_typephone_number
------- --------------- -------------------
46021HOME1111111111

Or really what I want in my export:

SELECT phone_number
FROM (SELECT phone_number
FROM employee_people_phones
WHERE emp_id = '46021'
ORDER BY decode(phone_type, 'HOME', 'a', 'WORK', 'b', 'CELL', 'c', 'z')) results
WHERE ROWNUM = 1

phone_number
-------------------
1111111111

However, when the Top-N query is added as a sub-select in a larger query using the employee id from the larger query (WHERE emp_id = export.emp_id), it fails saying that �export.emp_id� is not a valid id.

(SELECT phone_number
FROM (SELECT phone_number
FROM employee_people_phones
WHERE emp_id = export.emp_id
ORDER BY decode(phone_type, 'HOME', 'a', 'WORK', 'b', 'CELL', 'c', 'z')) results
WHERE ROWNUM = 1)

1.Any way around this? Is it possible to put a Top-N (with a WHERE clause using data from the main query) in a sub-select?

2.Any alternatives (other than Top-N) to delivering a ROWNUM=1 result with a �custom� ORDER BY statement?

Other Notes: Yes, we know we could do two queries in the data conversion first deliver the bulk data to the target table, and then update with the phone numbers. However, for multiple reasons, that is less than desirable.