Genesys Cloud - Main

I have a flow that specifies a Post Flow action that should execute immediately after the main flow ends. During testing I found that the main flow will end once the call disconnects which is the behavior I expect. However, we found many of the calls we forward to our partner call center do not execute this post flow action after disconnect. Our reports show it's only triggered about 20% of the time. What would be the reason for this inconsistency? 

Do you have Perform release link transfer turned ON in the Transfer to Number action? you might also want to check settings in the external trunk. The post flow should get triggered if the trunk was not released after transfer to a number. 

Thank you for the suggestion, I will look into these settings.

We discovered that when the receiver hangs up, the post flow is triggered. But when the caller hangs up, the post flow is not triggered.

You'd need to build some custom logic or a post call trigger to send a digital survey when the customer hangs up. You could also use the trigger to update a campaign that always running to call the customer back immediately to invite them back to a survey.