Dynamic form fields

Hi, i know is a very common problem this one but i have been searching
for information about the best way (rails way) of doing this and i
haven't met a good solution:

I have a model named Contact that has many phone numbers. I want to
have a form that dynamically add/delete 2 fields: phone number and a
category (home, personal, work, etc). I'm doing this with RJS, no
problem with that.

The question would be, is there a way of naming the fields and use
form_for :contact and send and hash/array of these lines and add them
as new relationships.

I read about parsing something like contact[phones][][number] but
haven't found much more info.

Thanks you in advance!