Yes it's possible with fritz.box as registrar and your dyndns-adress as sip server. You can ring the other phones but you can't hear each other. The issue has been reported by some people who already tried this. AVM didn't implement this on purpose they told me. It's only for internal calling, so remote registration works but the rtp audio won't be routed correctly.
You could use dtmfbox on your FritzBox to have it remotely connected. It's possible to install it seperately or with Freetz. Someone was asking for help in English in this thread in case you want to try dtmfbox with Freetz.
The only known Softphone that works correctly with this firmware issue is PhonerLite Beta if I'm not mistaken. The developer modified it so the audio will be routed correctly.
Hallo, as an AVM partner I visited the AVM Solutions Tour 2009 two weeks ago and was told there, that this function is on the wishlist of the developers, but there is no fixed release date planned. It was proposed, that perhaps after the next CeBit there would be time to implement this feature.