call_campaign.rb

class CallCampaign < ActiveRecord::Base
  has_many :campaign_targets, :autosave => true
  has_many :call_lists, :through => :campaign_targets

  has_many :phone_numbers, :through => :call_lists
end

call_list.rb

class CallList < ActiveRecord::Base
  attr_accessible :name
  has_many :call_list_memberships, :foreign_key => :call_list_id
  has_many :phone_numbers, :through => :call_list_memberships
end

call_list_membership.rb

class CallListMembership < ActiveRecord::Base
  attr_accessible :call_list_id, :phone_number_id
  belongs_to :call_list
  belongs_to :phone_number
end

phone_number.rb

class PhoneNumber < ActiveRecord::Base
  has_many :call_list_memberships
  has_many :call_lists, :through => :call_list_memberships
end