Belgacom I-Talk

Recently our telecom and adsl provider belgacom (belgium) started to offer VOIP services for end users.
I requested the service and manged to use it with a softphone under winXP (X-lite). After configuring the softphone I could both make calls and recieve calls.

Now i'm trying to configure my trixbox to be able te send and recieve calls via the voip service, i have tried allot of things but I do not realy have a clue what im doing.

The settings i used on the softphone xlite (these work!):
Display name: I-talk
User name: 016xxxxxx (the number i recieved form my provider)
Password: xyz
Auth name: u16xxxx (the auth. name i recieved from my provider)
Domain: voip.belgacom.be
Register with domain: yes
send outbound via proxy: voip.belgacom.be
using stun: stun.fwd.net
enable ice
(at first i get a 500 error but after a few seconds the phone works fine)

The settings i used on my trixbox
outbound caller id "myname"<016xxx>
trunk name:italk
host=voip.belgacom.be
nat=yes
secret=xyz
username=016xxxxxx
autname=u16xxxxx

no incoming settings configured (since i have absolutly no clue what needs to come here)

register string: 016xxxx:xyz:u16xxxxxx@voip.belgacom.be

Some more info i have gathered...
When i check the sip debug log see the folowing (only posted usefull parts):

SIP packets captures form a logon of the softphone
my local ip is 172.20.20.2, belgacom voip server is 81.24.24.38

172.20.20.2 81.240.251.38 SIP Request: REGISTER sip:voip.belgacom.be
...
CSeq: 1 REGISTER
Expires: 3600
...
81.240.251.38 172.20.20.2 SIP Status: 401 Unauthorized (1 bindings)
...
CSeq: 1 REGISTER
WWW-Authenticate: Digest realm="voip.belgacom.be",
nonce="4715e2ee061c5b7f3695afc5b2c23465b9c7b5f8",algorithm=MD5
...
172.20.20.2 81.240.251.38 SIP Request: REGISTER sip:voip.belgacom.be
...
CSeq: 2 REGISTER
Expires: 3600
Authorization: Digest username="u16652293",
realm="voip.belgacom.be",nonce="4715e2ee061c5b7f3695afc5b2c23465b9c7b5f8",
uri="sip:voip.belgacom.be",response="741f36dde3bbb816406982bc90e1828a",
algorithm=MD5
...
81.240.251.38 172.20.20.2 SIP Status: 200 OK (1 bindings)
... almost the same here expet Cseq goes up
172.20.20.2 81.240.251.38 SIP Request: REGISTER sip:voip.belgacom.be
81.240.251.38 172.20.20.2 SIP Status: 200 OK (0 bindings)
172.20.20.2 81.240.251.38 SIP Request: REGISTER sip:voip.belgacom.be
81.240.251.38 172.20.20.2 SIP Status: 500 Internal Server Error (1 bindings)
172.20.20.2 81.240.251.38 SIP Request: REGISTER sip:voip.belgacom.be
81.240.251.38 172.20.20.2 SIP Status: 401 Unauthorized (1 bindings)
172.20.20.2 81.240.251.38 SIP Request: REGISTER sip:voip.belgacom.be
81.240.251.38 172.20.20.2 SIP Status: 401 Unauthorized (1 bindings)
... at this point the softphone is logged in

SIP packets captures form a the trixbox
172.20.20.5 81.240.251.38 SIP Request: REGISTER sip:voip.belgacom.be
CSeq: 117 REGISTER
Expires: 120
Authorization: Digest username="u16652293",
realm="voip.belgacom.be", algorithm=MD5, uri="sip:voip.belgacom.be",
nonce="4715f1fdc2044f25c0f56abbd2b280a452263e9b",
response="70b9a1d602716deeae43a12967f23bcc", opaque=""
81.240.251.38 172.20.20.5 SIP Status: 423 Interval Too Brief (1 bindings)
...this goes on for some time...
172.20.20.5 81.240.251.38 SIP Request: OPTIONS sip:voip.belgacom.be
81.240.251.38 172.20.20.5 SIP Status: 403 Forbidden
172.20.20.5 81.240.251.38 SIP Request: REGISTER sip:voip.belgacom.be
CSeq: 102 REGISTER
Expires: 120
... no auth stuff here...
81.240.251.38 172.20.20.5 SIP Status: 401 Unauthorized (1 bindings)
172.20.20.5 81.240.251.38 SIP Request: REGISTER sip:voip.belgacom.be
CSeq: 103 REGISTER
Expires: 120
... auth stuff here...
81.240.251.38 172.20.20.5 SIP Status: 423 Interval Too Brief (1 bindings)
...and goes on like that with the REGISTER and the 423 error from the server...

more info
when i change the register string to the default username:password@viop.belgacom.be i get a method error after the first packet because somewhere in the auth string it sends the username and not the authname.
When i use authname:password@viop.belgacom.be i get an instant forbidden back since it uses it in the from an to fields.

My trixbox works fine i have other trunks configured (voipbuster), i have exentions configured like softphones and a cisco7960. All these works fine.
Natting and portforwarding is also configured correcty since i can connect with a phone from outside the network.
Also i have configured the routes that work well with my voipbuster trunk.
When i try to call using the belgacom trunk it says all circuits busy.
The trixbox panel is reporting the belgacom trunk but says its not registered.

Any help on how to correctly configure this trunk ?