No voicemail attached to any specific extension

Hi,
I am an * newbie running * 1.6 and have basic voicemail and extensions setup. I was able to call all extensions and access voicemail setup. However, if one extension was busy, the caller could never reach to that extension’s voicemail. I got error of NOTICE[2722]: chan_sip.c: 18978 handle_request_subscribe: Received SIP subscribe for peer without mailbox:
my extension config is:
[internal]
exten => _1XXX,1,NoOp()
exten => _1XXX,n,Dial(SIP/${EXTEN},15)
exten => _1XXX,n,GotoIf($["${DIALSTATUS}" = “BUSY”]?busy:unavail)
exten => _1XXX,n(unavail),Voicemail(${EXTEN}@default,u)
exten => _1XXX,n,Hangup()
exten => _1XXX,n(busy),VoiceMail(${EXTEN}@default,b)
exten => _1XXX,n,hangup()
exten => 700,1,VoiceMailMain()

and voicemail is as:

[default]
; Define maximum number of messages per folder for a particular context.
;maxmsg=50
1111 => 1111,John Doe , johndoe@google.com,johndoe@google.net,tz=eastern
2222 => 2222,Jane Doe,janedoe@yahoo.com,janedoe@yahoo.net,tz=eastern

thank you for your help!