The best method is to take a field that you want to emulate and follow it through the code ...

If you just need data collected for the customer without verification that belongs to the customer table ... follow the field for:
customers_fax

fax can be searched for in the Developer's Tool Kit to show where it is used throughout the Catalog and Admin ...

If you need something that is tested and belongs to the customers table, then use:
customers_telephone

phone can be searched for in the Developer's Tool Kit to show where it is used throughout the Catalog and Admin ...