Comment 1 for bug 52611

Have your tried to connect to asterisk console ? It gives some informations on the interactions with voicemail (connect with a lot of v).

I mean something like this:
 asterisk -rvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv
  == Parsing '/etc/asterisk/asterisk.conf': Found
  == Parsing '/etc/asterisk/extconfig.conf': Found
Asterisk 1.2.7.1, Copyright (C) 1999 - 2006 Digium, Inc. and others.
Created by Mark Spencer <email address hidden>
Asterisk comes with ABSOLUTELY NO WARRANTY; type 'show warranty' for details.
This is free software, with components licensed under the GNU General Public
License version 2 and other licenses; you are welcome to redistribute it under
certain conditions. Type 'show license' for details.
=========================================================================
Connected to Asterisk 1.2.7.1 currently running on obelix (pid = 24848)
Verbosity is at least 48
Jul 18 16:12:51 WARNING[24861]: chan_sip.c:3490 process_sdp: Unknown SDP media type in offer: video 5010 RTP/AVP 31
    -- Executing Set("SIP/0664-5682", "LANGUAGE()=fr") in new stack
    -- Executing VoiceMailMain("SIP/0664-5682", "") in new stack
    -- Playing 'vm-login' (language 'fr')
    -- Playing 'vm-password' (language 'fr')
    -- Playing 'vm-youhave' (language 'fr')
    -- Playing 'digits/1' (language 'fr')