extension.conf:

[myphones]
include => cme-pbx
include => voice-mail

[cme-pbx]
exten => _6XX,1,Dial(SIP/${EXTEN}@cme-pbx)
exten => _6XX,2,Playback(invalid)
exten => _6XX,3,Hangup
exten => _5600,1,Dial(SIP/601$,45)

[voice-mail]
exten => 5699,1,VoicemailMain(${CALLERIDNUM}$)
exten => 5699,2,Wait(3)
exten => 5699,3,Hangup
exten => t,1,Playback(vm-goodbye)
exten => t,2,Hangup
exten => _56XX,1,Answer
exten => _56XX,2,Voicemail(u${EXTEN:1})
exten => _56XX,3,Hangup


sip.conf:

[cme-pbx]
type=peer
canreinvite=no
host=192.168.17.1
qualify=yes


[601$]
type=friend
language=en
host=dynamic
dtmfmode=rfc2833
canreinvite=yes
mailbox=601@default
context=myphones
qualify=yes