Jersey Client - How to send List in a form with a POST request

I am using the jersey client to send POST requests to a web server. I have generally been building up a Form object with key-value pairs. However, I now have to send a List within my request. Here's an abridged version of my code

// Phone is a POJO consisting of a few Strings
public void request(List<Phone> phones) {
     Form form = new Form();
     form.add("phones", phones);
     ClientResponse response = WebService.getResponseFromServer(form);
     String output = response.getEntity(String.class);
     System.out.println(output);
}

public static ClientResponse getResponseFromServer(Form form) {
    Client client = createClient();
    WebResource webResource = client.resource(PATH);

    return webResource.type(MediaType.APPLICATION_FORM_URLENCODED).post(ClientResponse.class, form);
}

Unfortunately, this doesn't seem to work, and I get a 400 bad request error. When I send a request directly

{"phones":[{"areaCode":"217","countryCode":"01","number":"3812565"}]}

I have no problems. Thanks in advance!