extensions.conf

[internal]
;autofallthrough=no
exten => 333,1, Dial(SIP/333,60,tr)
exten => 666,1, Dial(SIP/666,60,tr)


sip.conf


[general]
register => 333:pwd@server
register => 666:pwd@server
[666]
username=666
type=friend
secret=pwd
record_out=Adhoc
record_in=Adhoc
qualify=yes
port=5060
nat=yes
mailbox=666@device
host=dynamic
dtmfmode=rfc2833
context=internal
canreinvite=no
callerid=device <666>
[333]
username=333
type=friend
secret=pwd
record_out=Adhoc
record_in=Adhoc
qualify=yes
port=5060
nat=route
mailbox=333@device
host=dynamic
dtmfmode=rfc2833
disallow=all
context=internal
canreinvite=no
callerid=device <333>
allow=alaw