asterisk users - Aug 2006 - "Off-circuits are busy now. Please try your call again later"

I set up trixbox as described in Nerd Vettles howto (more or less). At 
hte moment only a SIP line. There are 2 extensions. I can call from ext. 
20 to 21 and vice cersa w/o problems.
When making a call now I get the message: "Off-circuits are busy now. 
Please try your call again later"
My outbound route looks like this:
9|.
XXXXXXXXXX
XXXXXXXXXXX
XXXXXXXXXXXX
XXXXXXXXXXXXX

and sip_additional.conf like this:

register=<myusername>:<mypassword>@sipgate.de/<myusername>

[20]
username=20
type=friend
secret=1234
record_out=Adhoc
record_in=Adhoc
qualify=yes
port=5060
nat=yes
mailbox=20
host=dynamic
dtmfmode=rfc2833
context=from-internal
canreinvite=no
callerid=Office <20>

[5550873]
username=<myusername>
secret=<mypassword>
nat=yes
insecure=very
host=sipgate.de
fromuser=<myusername>
fromdomain=sipgate.de
disallow=all
canreinvite=no
allow=ulaw

[Sipgate-in]
username=<myusername>
type=peer
secret=<mypassword>
nat=yes
insecure=very
host=sipgate.de
fromdomain=sipgate.de
dtmfmode=rfc2833
disallow=all
canreinvite=no
allow=ulaw

Any idea what I am doing wrong here?
Thank you very much for your help in advance