Use paging with the Twilio API to cycle through all of your phone numbers. Filter the results by the app sid associated with it.

findAppSid.js

//Cycle through all phone numbers, and filter on appsid
//you can't query by Appsid via the API, so getting all numbers and iterating is the way to do it.
//Module
//npm install request async
var request = require('request');
var async = require('async');

//Twilio account variables
var sid = 	'ACxxxx';
var auth = 	 '52xxxxxx';
var appSid = 'APxxxxxx';   //appsid for a phone number you are looking for.

var baseUrl = 'https://api.twilio.com/2010-04-01/Accounts/' + sid + '/IncomingPhoneNumbers.json?PageSize=1000&Page=0';

var auth = {
	user: sid,
	pass: auth
}

var parameters = {}

var count = 0;
pullNumbers(baseUrl);

function pullNumbers(uri) {
	parameters = {
		url: uri,
		auth: auth,
	}

	request.get(parameters, function(error, httpResponse, body){
				//don't really need async, but if you call pullNumbers with
	      async.forEachOf(JSON.parse(body).incoming_phone_numbers, function(item,key){
						count = count +1;
						if (item.voice_application_sid == appSid) {
	          		console.log( count + " found appsid  " +  item.phone_number + " appsid = " + item.voice_application_sid ) ;
					 	}
	      });

	      if(JSON.parse(body).next_page_uri){
					console.log("getting nextpage, count = " + count);
	  			pullNumbers('https://api.twilio.com' + JSON.parse(body).next_page_uri);
	  		}
	});
}