As per the documentation, this service only allows you to retrieve the first 2,000 results. You can however look at breaking and filter based on category set. In most cases you would end up being able to retrieve all the POI's, with the odd case where there is more than 2,000 of the same POI type in the same area.
What is your business case that requires retrieving all these POI's? There are some other services we are investigating which might better suit your needs in the future.