Symfony Check if at least one of two fields isn't empty on form validation

I've been turning this around in my head for quite a while now and still wasn't able to find a solution to my problem. Using Symfony 4 forms and constraints I'm unable to setup a check to say that at least one of two fields must not be empty when submitting form that contains a sub-form.

I have a Booking entity which contains a Visitor entity which has a phoneNumber property and a email property. I'd like to be able to create a Booking which has a "visitors" CollectionType (where I'm allowed to add visitors from the BookingType form).

My BookingType form (a bit simplified):

class BookingType extends AbstractType
{
    private $router;
    private $translator;

    public function __construct(UrlGeneratorInterface $router, TranslatorInterface $translator)
    {
        $this->router = $router;
        $this->translator = $translator;
    }

    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->add('bookableTimeSlot', EntityType::class, [
                'label' => 'entity.booking.bookable-time-slot',
                'class' => BookableTimeSlot::class,
                'choice_label' => function ($bookableTimeSlot) {
                    return $bookableTimeSlot->getStartDateTime()->format('d.m.Y h\hi');
                }
            ])
            ->add('visitors', CollectionType::class, [
                'entry_type' => VisitorType::class,
                'label' => 'entity.booking.visitors',
                'allow_add' => true,
                'by_reference' => false,
                'entry_options' => ['label' => false]
            ])
        ;
    }

    public function configureOptions(OptionsResolver $resolver)
    {
        $resolver->setDefaults([
            'data_class' => Booking::class,
            'user' => User::class,
        ]);
    }
}

My Visitor entity (a bit simplified):

<?php

namespace App\Entity;

use Doctrine\ORM\Mapping as ORM;
use Symfony\Component\Validator\Constraints as Assert;
use Symfony\Component\Validator\Context\ExecutionContextInterface;

/**
 * @ORM\Entity(repositoryClass="App\Repository\VisitorRepository")
 */
class Visitor
{
    /**
     * @ORM\Id()
     * @ORM\GeneratedValue()
     * @ORM\Column(type="integer")
     */
    private $id;

    /**
     * @ORM\Column(type="string", length=255)
     */
    private $firstName;

    /**
     * @ORM\Column(type="string", length=255)
     */
    private $lastName;

    /**
     * @ORM\Column(type="string", length=45, nullable=true)
     */
    private $phone;

    /**
     * @ORM\ManyToOne(targetEntity="App\Entity\Booking", inversedBy="visitors")
     * @ORM\JoinColumn(nullable=false)
     */
    private $booking;

    /**
     * @ORM\Column(type="string", length=255, nullable=true)
     */
    private $email;

    public function getId(): ?int
    {
        return $this->id;
    }

    public function getFirstName(): ?string
    {
        return $this->firstName;
    }

    public function setFirstName(string $firstName): self
    {
        $this->firstName = $firstName;

        return $this;
    }

    public function getLastName(): ?string
    {
        return $this->lastName;
    }

    public function setLastName(string $lastName): self
    {
        $this->lastName = $lastName;

        return $this;
    }

    public function getPhone(): ?string
    {
        return $this->phone;
    }

    public function setPhone(string $phone): self
    {
        $this->phone = $phone;

        return $this;
    }

    public function getBooking(): ?Booking
    {
        return $this->booking;
    }

    public function setBooking(?Booking $booking): self
    {
        $this->booking = $booking;

        return $this;
    }

    public function getEmail(): ?string
    {
        return $this->email;
    }

    public function setEmail(?string $email): self
    {
        $this->email = $email;

        return $this;
    }
}

And finaly my VisitorType form (a bit simplified):

class VisitorType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->add('firstName', TextType::class, [
                'label' => 'entity.visitor.first-name',
            ])
            ->add('lastName', TextType::class, [
                'label' => 'entity.visitor.last-name',
            ])
            ->add('phone', TextType::class, [
                'label' => 'entity.visitor.phone-number',
                'required' => false,
            ])
            ->add('email', TextType::class, [
                'label' => 'entity.visitor.email',
                'required' => false,
                'constraints' => [
                    new Email()
                ]
            ])
        ;
    }

    public function configureOptions(OptionsResolver $resolver)
    {
        $resolver->setDefaults([
            'data_class' => Visitor::class,
        ]);
    }
}

I've tried to add an Expression constraint to the email and phone field which looked something like this:

new Expression([
    'expression' => 'this.getPhone() == null && this.getEmail() == null'
])

Also tried to add constraint directly to the entity, but nothing seems to work correctly for me.

Any help would be greatly appreciated.

UPDATE I haven't specified this, but my problem comes from the fact that I would like to validate the VisitorType form from another form which adds the VisitorType as a CollectionType.