IOS subscription end notification

I am looking to build a ios application that will use an auto-renewable subscription on a monthly biases. We will be using some VOIP features that we will be charged monthly for. My question is, how can we be notified when a user cancels a subscription in IOS, so that we can cancel the users VOIP telephone number with our provider.

You will need to verify the receipt the same way you do for regular In App Purchases, but obviously you will do this every time the subscription period expires. Check out the documentation on this, Verifying an Auto-renewable Subscription Receipt

So based on this, it appears you will need to setup your server to check for a valid subscription every month (cron job?). This will allow you to cancel the VOIP number in your system if the subscription is no longer valid. Your app would be responsible for verifying a subscription is still valid when it is opened or the user attempts to access features that require a subscription.

Updated For Comment

The sandbox is supposed to return expiration dates in the following scheme.

This is from page 164 of the iTunes Connect Developer Guide

If it is not working this way you may want to file a bug report.