app/views/phones/_phone_form.html.haml
.row
.col-12
%label Phone numbers
#phones
= f.fields_for :phones do |phone|
= render 'phones/phone_fields', f: phone
.links
= link_to_add_association 'Add number', f, :phones,
partial: 'phones/phone_fields', class: 'btn btn-secondary btn-sm'
%br/