_form.html.erb

<%= form_for([@customer, @phone]) do |f| %>
  <% if @phone.errors.any? %>
    <div id="error_explanation">
      <h2><%= pluralize(@phone.errors.count, "error") %> prohibited this phone from being saved:</h2>

      <ul>
      <% @phone.errors.full_messages.each do |msg| %>
        <li><%= msg %></li>
      <% end %>
      </ul>
    </div>
  <% end %>

  <div class="field">
    <%= f.label :number %><br />
    <%= f.number_field :number %>
  </div>
  <div class="field">
    <%= f.label :type %><br />
    <%= f.number_field :type %>
  </div>
  <div class="actions">
    <%= f.submit %>
  </div>
<% end %>

customer.rb

class Customer < ActiveRecord::Base
  has_many :phones

  accepts_nested_attributes_for :phones, :allow_destroy => true
end

phone.rb

class Phone < ActiveRecord::Base
  belongs_to :customer

  attr_accessible :number, :type
  #validates :number, presence: true, length: { is: 10 }
end

phones_controller.rb

class PhonesController < ApplicationController
  # GET /phones
  # GET /phones.json
  def index
    @phones = Phone.all

    respond_to do |format|
      format.html # index.html.erb
      format.json { render json: @phones }
    end
  end

  # GET /phones/1
  # GET /phones/1.json
  def show
    @phone = Phone.find(params[:id])

    respond_to do |format|
      format.html # show.html.erb
      format.json { render json: @phone }
    end
  end

  # GET /phones/new
  # GET /phones/new.json
  def new
    @phone = Phone.new
    @customer = Customer.find(params[:customer_id])

    respond_to do |format|
      format.html # new.html.erb
      format.json { render json: @phone }
    end
  end

  # GET /phones/1/edit
  def edit
    @phone = Phone.find(params[:id])
  end

  # POST /phones
  # POST /phones.json
  def create
    @phone = Phone.new(params[:phone])
    @customer = Customer.find(params[:customer_id])

    respond_to do |format|
      if @phone.save
        format.html { redirect_to @customer, notice: 'Phone was successfully created.' }
        format.json { render json: @phone, status: :created, location: @phone }
      else
        format.html { render action: "new" }
        format.json { render json: @phone.errors, status: :unprocessable_entity }
      end
    end
  end

  # PUT /phones/1
  # PUT /phones/1.json
  def update
    @phone = Phone.find(params[:id])

    respond_to do |format|
      if @phone.update_attributes(params[:phone])
        format.html { redirect_to @customer, notice: 'Phone was successfully updated.' }
        format.json { head :no_content }
      else
        format.html { render action: "edit" }
        format.json { render json: @phone.errors, status: :unprocessable_entity }
      end
    end
  end

  # DELETE /phones/1
  # DELETE /phones/1.json
  def destroy
    @phone = Phone.find(params[:id])
    @phone.destroy

    respond_to do |format|
      format.html { redirect_to @customer }
      format.json { head :no_content }
    end
  end
end