First I tried without STUN, and got the following trace:

U 2008/07/22 21:03:48.983041 1.1.210.76:57501 -> 1.1.232.18:5060
REGISTER sip:test.nl:5060 SIP/2.0.
Via: SIP/2.0/UDP 172.31.0.55:5060;branch=z9hG4bK57489228aa596f71.
Max-Forwards: 70.
To: <sip:ldr-line1@test.nl>.
From: <sip:ldr-line1@test.nl>;tag=xETN4EDMxED.
Call-ID: 928BD3011481111@172.31.0.55.
CSeq: 2 REGISTER.
Contact: <sip:ldr-line1@172.31.0.55:5060>.
Content-Length: 0.
Expires: 3600.