ubuntu*CLI> core show channel SIP/6004-00000001 
 -- General --
           Name: SIP/6004-00000001
           Type: SIP
       UniqueID: 1372708831.1
       LinkedID: 1372708831.1
      Caller ID: 6004
 Caller ID Name: 6004
Connected Line ID: (N/A)
Connected Line ID Name: (N/A)
    DNID Digits: 6001
       Language: en
          State: Up (6)
          Rings: 0
  NativeFormats: 0x200000000 (speex16)
    WriteFormat: 0x40 (slin)
     ReadFormat: 0x40 (slin)
 WriteTranscode: Yes slin->g722->slin16->speex16
  ReadTranscode: Yes speex16->slin16->g722->slin
1st File Descriptor: 16
      Frames in: 916
     Frames out: 300
 Time to Hangup: 0
   Elapsed Time: 0h0m20s
  Direct Bridge: <none>
Indirect Bridge: <none>
 --   PBX   --
        Context: internal
      Extension: 6001
       Priority: 2
     Call Group: 0
   Pickup Group: 0
    Application: VoiceMail
           Data: 6001
    Blocking in: ast_waitfor_nandfds
      Variables:


root@ubuntu:/var/lib/asterisk/sounds# file /var/spool/asterisk/voicemail/default/6001/INBOX/msg0001.WAV 
/var/spool/asterisk/voicemail/default/6001/INBOX/msg0001.WAV: RIFF (little-endian) data, WAVE audio, GSM 6.10, mono 8000 Hz