many_to_many.rb

class Campaign
  field :name
  # ...

  has_many :shortcodes_services
  has_many :phone_numbers, through: :campaigns_phone_number
end

class CampaignsPhoneNumber
  belongs_to :campaign
  belongs_to :phone_number
end

class PhoneNumber
  field :name
  field :phone_number
  # ...
  
  has_many :shortcodes_services
  has_many :campaigns, through: :campaigns_phone_number
end

3.times { phone_numbers << PhoneNumber.create(name: 'hello world', phone_number: '1234') }
campaign = Campaign.create(name: 'hello world', phone_numbers: phone_numbers)

# In a later stage
campaign.update(phone_numbers: phone_numbers.last)