extensions.conf:

[web-default]
exten => myiaxno,1,Answer()
exten => myiaxno,n,MixMonitor(${STRFTIME(${EPOCH},,%Y%m%d-%H%M%S)}-${CALLERID(num)}.wav)
exten => myiaxno,n,Dial(IAX2/web-matt)
exten => myiaxno,n,Voicemail(mbox141@web)
exten => myiaxno,n,HangUp()

(have also tried :
exten => myiaxno,101,Voicemail(mbox141@web)
exten => myiaxno,102,HangUp()


voicemail.conf

[web]
mbox141 => 364,My name,mail@email.info,,



cli output :-
Asterisk 1.4.13, Copyright (C) 1999 - 2007 Digium, Inc. and others.
Created by Mark Spencer <markster@digium.com>
Asterisk comes with ABSOLUTELY NO WARRANTY; type 'core 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 'core show license' for details.
=========================================================================
  == Parsing '/etc/asterisk/extconfig.conf': Found
Connected to Asterisk 1.4.13 currently running on 
ions*CLI>
Verbosity is at least 23
    -- Accepting UNAUTHENTICATED call from softiaxip:
    -- Executing [myiaxno@web-default:1] Answer("IAX2/softiaxip:4569-1", "") in new stack
    -- Executing [myiaxno@web-default:2] MixMonitor("IAX2/softiaxip:4569-1", "20071028-202500-callingno.wav") in new stack
    -- Executing [myiaxno@web-default:3] Dial("IAX2/softiaxip:4569-1", "IAX2/web-matt") in new stack
  == Begin MixMonitor Recording IAX2/softiaxip:4569-1
    -- Called web-matt
    -- Call accepted by ipaddy (format ilbc)
    -- Format for call is ilbc
    -- IAX2/web-matt-3 is ringing
    -- IAX2/web-matt-3 answered IAX2/softiaxip:4569-1

[Oct 28 20:25:02] WARNING[11586]: chan_iax2.c:7450 socket_process: Call rejected by ipaddy: Call rejected manually.
  == Spawn extension (web-default, myiaxno, 3) exited non-zero on 'IAX2/softiaxip:4569-1'
    -- Hungup 'IAX2/softiaxip:4569-1'
  == End MixMonitor Recording IAX2/softiaxip:4569-1