In this case it looks it has set ${HANGUPCAUSE} to 0 but it is actually 91, it is receiving the 91 just doing something funny with it. It would be nice to get it to work so the end user could receive the correct tone played to them.
As far as i know, asterisk isnt setting the pricause.
The pricause differs from country to country, even from provider to provider (you can google your local standard causes).
So you need to set the cause at the right place to the right value.
Also, a busy should be set, and if it is by
exten=> 5555,102,busy
exten=> 5555,103,congestion
If you let him run into the mailbox, you (of course) dont set busy at all (now you are answering the call technically)