If you've installed the aastra scripts, you may have experienced a few issues with them.  One common problem is the visual voicemail fails to authenticate on returning from a call. 


To resolve, simply find the cache path in the include/AastraCommon.php file and ensure it exists and Asterisk has write/read capability.  In my case,

mkdir /var/cache/aastra
chown asterisk:asterisk /var/cache/aastra -R

This resolves the authentication issue.