Just wanted to post this bit of info that's taken many hours to resolve! - hope it's useful to someone.
If you're using a cisco 7960 / 7940, behind a netgear DG834G router, you may experience problems even if it's all configured properly.
I found that I could make calls but not receive them, I phone tech support (who were amazingly helpful) couldn't fix it there and then... however they did mention that there was a problem with some versions of the router.
So basically if you've got a v1 or v2 834G then it won't work (even if you make the outbound proxy unprovisioned) - you need a v3+ - then it works like a dream!
Please let me know if you found that helpful