No additional hardware required if you are using Nokia e71 as a VoIP adapter.You will need to setup your extension with nat=yes and in an sip_nat.conf, you'll need to provide a DynDNS domain name (or any service with dynamically-updated IP address and a client) for "fromdomain," set the nat to yes, and provide your localnet, such as 192.168.1.0/255.255.255.0.
(Note: to prevent the # to turn into a numbered list, I used a vertical bar to make the # stay as-is. Just remove the | when implementing the code into the sip_nat.conf file.
Here's the website for DynDNS: www.dyndns.org
Then, you should be able to connect remotely by using a domain name. I'm not sure if I'm explaining right, but I hope this helps.