How to retrieve under checkout > payment phone optional value using Shopify Functions (cart and checkout validation)

[ghost]

Hi everyone,
i want to retrieve and check if the 'phone (optional)' value is provided via the Shopify Functions cart and checkout validation on checkout > payment section, but when i use the graphQl RunInput with:

query RunInput {
  cart {
    cost {
      subtotalAmount {
        amount
      }
    }
    deliveryGroups {
      deliveryOptions {
        title
      }
      deliveryAddress {
        name
        phone
      }
    }
  }
}

For trying to retrieve phone value, there is the following TypeScript code pat

  const phoneNumber = input.cart.deliveryGroups
    .flatMap((deliveryOptions) => deliveryOptions.deliveryAddress?.phone)

The phone value is always null, someone has an idea please because according to the doc
deliveryAddress: MailingAddress can be used for billing address.

Thank you