I got it working sitting behind of firewall - but the volume of the handset won't save, and the volume control buttons don't work properly. So everytime we answer a call currently with the Snom 300, we can't hear the person on the other end!
Took the query to Snom Support.... and I'm awaiting an answer!
Our first phone was a Grandstream 2000, but is actually quite good. Seems to un-register too frequently though!