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,,