Queue or virtual extension is not reachable from "outsi

We are trying to setup an Asterisk Queue
Its working fine inside our office, but there is no route to destination when trying to get from outside.
The Queue has an extension in extensions.conf, but has no entry in SIP.conf.
The problem we face is: We cant reach the Queue from outside, because we dont have a SIP phone with this number.
How is it possible to call the 40 extension from outside?

Our Extension.conf

[pbx2test]

exten = 500,1,Dial(SIP/500,20)
exten = 500,n,Playback(vm-nobodyavail)
exten = 500,n,Hangup()

exten = 20,1,Dial(SIP/501,20)
exten = 20,n,Playback(vm-nobodyavail)
exten = 20,n,Hangup()

; Warteschlange - Test

exten => 40,1,Answer()
exten => 40,n,Set(CHANNEL(mussicclass)=default)
exten => 40,n,Queue(testschlange,t)

include => digital-dialout
include => digital-dialin

[digital-dialin]

exten => _X.,1,Set(Rufnummer=${EXTEN:-2:2}) ; Use the last 2 Digits to place a call
exten => _X.,n,SayDigits(${Rufnummer}) ; Test the Digits
exten => _X.,n,Dial(SIP/${Rufnummer}) ; Call the SIP Extension

[digital-dialout]

exten => _0X.,1,Dial(PIKA/digital/g0/${EXTEN:1},30) ; Dial on PIKA T1/E1 trunk
exten => _0X.,n,Congestion(5)

Our Sip.conf

[general]

bindport = 5060
bindaddr = 172.16.0.41
;disallow = all
;allow = alaw

[500]
username = 500
context = pbx2test
type = friend
secret = 1234
callerid = "pbxtest1"
host = dynamic

[20]
username = 20
context = pbx2test
type = friend
secret = 1234
callerid = "pbxtest2"
host = dynamic

try

exten => _X.,n,Goto(pbx2test,40,1)

Should work.

Ian

Thanks alot, working fine !