The Trixbox Wiki has a number of digestible pages of advice on how to successfully deploy a VoIP application. Here are recommendations for remote sites.

Formula for the best remote telecommuter Experience

1. Use T1 internet access at the main location, not DSL or Cable.It’s worth the additional expense in order to ensure good, steady performance at your main location.
2. If your routers and/or firewalls support QoS features, activate them. Give priority to the SIP and RTP protocols. Consider replacing equipment that lacks VoIP-aware QoS features. See Also: How do I use QoS on my network?
3. Consider using one of our Suggested Routers with QoS on both ends of your connection.
4. If your QoS solution allows you to limit total bandwidth, set the limit to slightly less than the line speed of your internet connection. Use a DSL line speed test to determine where you should set your limits. Setting it about 5-10 Kb below your maximum speed will keep the packet buffers from filling up on your DSL/Cable modem. This will yield better overall performance.
5. Consider having two internet connections… one for your existing data application, and one for your VOIP phone and trixbox Pro servers. You can use this approach in your main location, as well as your remote locations. If you use this approach, you may not need any QoS capable equipment.
6. If possible, connect your main office and your remote office using the same internet provider. Usually performance on the same provider’s network is superior to the performance when traffic needs to traverse multiple internet backbone networks.
7. If possible, remove NAT devices between the trixbox Pro system, and the remote telecommuters.
8. If you must use a NAT configuration, consider using a “DMZ Host/Server” configuration rather than port forwarding. This uses less CPU power in the router/firewall and yields optimal performance.

   1. At the main location, the setting will forward all unknown packets to your trixbox Pro server.
   2. At the remote locations, the setting will forward all unknown incoming packets to the IP Phone.
   3. Reserve the phone’s IP address in DHCP or give the phone a static IP Address on your private network in the remote location so the IP Address does not change. If you use a static IP Address, pick one outside of your dynamic DHCP IP Address range.

9. For mission critical remote employees, consider using a fractional T1 internet service at the remote office instead of a Cable/DSL connection.