Write specs of Phone model
user
```ruby
require "spec_helper"

describe Phone do
  it "does not allow duplicate phone numbers per contact" do
    contact = Contact.create(
                             firstname: "Joe",
                             lastname: "Tester",
                             email: "joetester@example.com"
                            )
    contact.phones.create(
                          phone_type: "home",
                          phone: "123-456-789"
                         )
    mobile_phone = contact.phones.build(
                                        phone_type: "mobile",
                                        phone: "123-456-789"
                                       )
    expect(mobile_phone).to have(1).errors_on(:phone)
  end

  it "allows two contacts to share a phone number" do
    contact = Contact.create(
                             firstname: "Joe",
                             lastname: "Tester",
                             email: "joetester@example.com"
                            )
    contact.phones.create(
                          phone_type: "home",
                          phone: "123-456-789"
                         )
    other_contact = Contact.new
    other_phone = other_contact.phones.build(
                                             phone_type: "home",
                                             phone: "123-456-789"
                                            )
    expect(other_phone).to be_valid
  end
end
```
assistant