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,
chown asterisk:asterisk /var/cache/aastra -R
This resolves the authentication issue.