Firewalls and VOIP

Got VOIP? And if you are a company that deals with the internet most probably, you got a firewall too! Could it be a Cisco PIX? Well fear no more.
You can configure the PIX to allow the VOIP traffic and still keep your network safe.
Cisco itself is offering a sample configuration for PIX 500 series Firewalls.
The sample configuration, a PIX firewall is configured to allow the traversal of two different Voice over IP (VoIP) protocols, H.323, and Session Initiation Protocol (SIP). Due to the fact that VoIP protocols are made up of signaling and IP address/port combinations, there are a number of issues with VoIP and Network Address Translations (NAT). The PIX firewall fixup protocol addresses these issues.
Here is the document