WP Rest API - Post to ACF field?

I'm trying to post to my custom Wordpress field via the REST API from my android app. That said, when I look at the JSON structure of any ACF fields, they're nested inside "acf" like so:

{
  "acf": {
    "phone_number": "000-0000"
  }
}

I'm trying to post a phone number to the phone_number field at my endpoint with the following code/structure, but it won't seem to save?

        OkHttpClient client = new OkHttpClient();

        String url = "http://myurl.com/wp-json/wp/v2/users/loggedinuser/36";

        String bearer = "Bearer ";

        MediaType JSON = MediaType.parse("application/json; charset=utf-8");
     
         JSONObject jsonObject = new JSONObject();
            try {

            jsonObject.put("phone_number", "777-348-4349");
                
            } catch (JSONException e) {
                e.printStackTrace();
            }

        RequestBody body = RequestBody.create(JSON, jsonObject.toString());
        Request request = new Request.Builder()
                .url(mergeUrl)
                .method("POST", body)
                .addHeader("Accept-Charset", "application/json")
                .addHeader("Authorization", bearer + userToken)
                .addHeader("Content-Type", "application/json")
                .build();

        Response response = null;

        try {
            response = client.newCall(request).execute();

            String resStr = response.body().string();

            int responseCode = response.code();

            if (responseCode == 200) {


            } else {

            }

        } catch (IOException | JSONException e) {
            e.printStackTrace();
        }

        return null;
    }}

I imagine it's because phone_number is nested inside of "acf". How can I write this line:

jsonObject.put("phone_number", "777-348-4349");

so that it matches the structure above? I can't seem to figure out how to nest phone_number inside of the acf value.