SIP, NAT and Speedtouch

I've recently been playing with ENUM for (some of) my VoIP calls, and it struck me that it would be a good idea to let SIP calls into my network in order to allow incoming calls.  It would also potentially allow me to connect as an extension from wherever I happen to be, as my phone (a Nokia N95) has VoIP functionality, and indeed already works as an extention while I'm in the house.

So... I'm looking at how to get SIP to traverse the network.  Now, the ADSL router in question is a Speedtouch 716v5.  In theory it's a simple affair to just set up Application Sharing to point th relevant ports through to my server.

Unfortunately, it's not working, and I'm currently struggling as to trying to find out where the problem actually IS.  Now, I run several subnets, and the asterisk server isn't on the same one that the router is.  They can talk, as the routing is all set up, but it might be an issue.  It certainly ensured that the Actiontec router on the other line refused to accept port-forwarding to anything other than addresses on the direct LAN connection.

I'll keep you posted...

Edit:  Well, setting up application sharing for SSH, pointed at the asterisk server, works just fine. So, it's something to do with SIP itself.  Probably due to the speedtouch having SIP capabilities itself.  

Edit: OK no it's not.  I had an error in the routing tables in the OpenWRT box that joins all the subnets together..  What I have found, just watching the logs on a softphone and in asterisk, is that the SIP register requests re getting through to the asterisk server, and it's sending out repsonses, apparently even to the correct IP address.  Unfortunately the 'phone isn't receiving them.  the Conntrack table conuters in OpenWRT show packets are being sent in both directions, though. Hmmm..