SBC Cheatsheet:
Why Firewalls Cause Problems

When deploying Remote Phones, whether as a Carrier in a hosted solution or as an Enterprise with home or remote employees, these phones will have to reside behind some type of firewall. Firewalls are meant to provide security by hiding the private network addresses of end user devices and at the same time preventing all unsolicited network traffic from entering the private network. In addition, VoIP uses SIP (RFC 3261) as a communications protocol, which is an application and not a network routing protocol, and firewalls don’t provide application support.