[internal]
exten => 101,hint,SIP/station1
exten => 101,1,Dial(SIP/station1)
exten => 102,hint,SIP/station2
exten => 102,1,Dial(SIP/station2)
exten => 3500,1,VoiceMailMain()
exten => xxxxxx,hint,SIP/fwd
exten => xxxxxx,1,Dial(SIP/xxxxxx@fwd)
exten => xxxxxx,2,Playback(invalid)
exten => xxxxxx,3,Hangup

sip.conf

[general]
disallow=all
allow=ulaw
port=5060
bindaddr=0.0.0.0
externip=xxx.xxx.xxx.xxx
localnet=192.168.0.0/255.255.255.0
maxexpirey=180
defaultexpirey=160
srvlookup=yes

register => xxxxxx:yyyyyy@fwd.pulver.com/xxxxxx

[station1]
disallow=all
allow=ulaw
allow=alaw
type=friend
context=internal
host=dynamic
dtmfmode=rfc2833
callerid="101" <101>
nat=no
qualify=yes
canreinvite=no
notifyringing=yes
notifyhold=yes
call-limit=99

[station2]
disallow=all
allow=ulaw
allow=alaw
type=friend
context=internal
host=dynamic
dtmfmode=rfc2833
callerid="102" <102>
nat=no
qualify=yes
canreinvite=no
notifyringing=yes
notifyhold=yes
mailbox=1234
call-limit=99