Problems with incoming calls & Asterisk

I've got the following set-up going now:
Asterisk <-> NAT router <-> internet
siproxd is running on the router. I've configured Asterisk like this in 
sip.conf:

[general]
outboundproxy=192.168.0.1
outboundproxyport=5060

register => julius:pass@<router (siproxd) ip>

for all the users I need

Outgoing calls all work perfectly with this set-up. Also most incoming 
calls are working fine, but not always though. Looking at the logs, it 
seems the party trying to call me, sometimes is handed the internal IP 
address of my Asterisk box. This results in voice going only one way. 
Any idea what could be going wrong here?
If you want to test this, you can call me on 
sip:[email protected]

Thanks a lot in advance,
Julius

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