How to configure the Passthru applet after hangup?  We would like to have a call back to our application after the user hangs up the phone, to identify exactly at which step the user hangs up.

To uniquely identify a call use the CallSid parameter that comes to you from passthru applet

There are two parts to this: 

1) When the flow moves from one applet to the next. You will need to put a passthru applet in every "next" section. Something like this: 

Greeting --> passthru --> Call connect --> passthru --> voicemail --> passthru

This is going to slow down your call flow quite a bit. Read Is Passthru slowing my call flow to understand the repercussions. 

2) When the call moves from one agent to another within the Connect applet. You will need to use the "During the call" option in the connect applet and accept requests according to agent passthru specs

To identify which stage the user dropped off, you could now keep a counter (in your db for this callSid) that keeps incrementing every time you get a ping from exotel. There may be other algorithms too.