Voice over IP overview
VoIP or Internet Telephony is a NGN-appliance and maybe one day will totally replace telephony over POTS. A couple of VoIP-protocols are available, e.g. SIP.
a VoIP account at some VoIP provider
VoIP hardware (e.g. a SIP telephone) or/and VoIP software
on your PC
The OpenWrt repository contain a couple of software packages, to enable you to not only make your VoIP work through your NAT, but also do "cool stuff" with it:
Hardening Your System