Regarding the phone not ringing on incoming it may be because the phone is set to only re-register every 1 hour and the NAT binding is timing out. Set it to re-register maybe every 5 mins or less and retest.
If you are getting no audio this could be a port blocking or NAT issue. The above suggestion might fix this.
What router are you using and which ISP?
The above post has been made by an ISP REPRESENTATIVE (although not necessarily the ISP being discussed in the post).