Accept servers send the callbacks instantly after performing a transaction, whether it was test or live transaction, your server should receive both transaction processed and response callback, check this link in case you're confused about the difference between both.
In order to test the callbacks sent from Accept's endpoint, you might use one of these HTTP testing tools [ "Webhook", "RequestBin", "RequestWatch", ..etc ] or any other preferred HTTP testing tool, by which you can inspect the received callbacks.
So now you've created your temporary testing endpoint by using one of the mentioned tools or any other similar tool , now please add the URL of this endpoint in your transaction processed callback and response callback in your test integration ID, then perform a transaction.
Now you have the sent callbacks on your endpoint, so you can start debugging your endpoint in a better way. Also, you might want to check if your server allowing receiving python requests or not, sometimes this might be the problem.
If you have any further inquiries kindly assign a ticket to Accept's technical support team on "firstname.lastname@example.org".