rest/V1/guest-carts/:cartId/billing-address

rest/V1/mine/:cartId/billing-address

{
    "id": 330,
    "region": null,
    "region_id": null,
    "region_code": null,
    "country_id": "IN",
    "street": [
        "1st cross street"
    ],
    "telephone": "1234567890",
    "postcode": null,
    "city": "Hyderabad",
    "firstname": "Halll",
    "lastname": "test",
    "email": null,
    "same_as_billing": 0,
    "save_in_address_book": 0
}

{
        "id": 330,
        "region": null,
        "region_id": null,
        "region_code": null,
        "country_id": "IN",
        "street": [
            "1st cross street"
        ],
        "telephone": "1234567890",
        "postcode": null,
        "city": "Hyderabad",
        "firstname": "Halll",
        "lastname": "test",
        "email": null,
        "same_as_billing": 0,
        "save_in_address_book": 0,
        "extension_attributes": {
         "city_id": "1"
        }
    }

/**
 * {@inheritDoc}
 */
public function get($cartId)
{
    $cart = $this->quoteRepository->getActive($cartId);
    return $cart->getBillingAddress();
}