Cisco Router Firewall to work with VoIP

We will be implementing Cisco routers for all our VoIP installations. The setup is with a Hosted softPBX outside the organization. Previously we had to research the organizations existing firewall box to see if it was cabable of handling the NAT process for VoIP phones. Now we want to use the existing firewall security capabilities of Cisco 800, 1800, and 2800 series routers to handle this NAT process for VoIP calls.

1. What is this "NAT" functionality that handles VoIP calls outside the organization (though a firewall) called? I don't believe it's NAT(something about packet rewrite and handling support of codecs like G.711).

2. Do all these series routers firewallls handle this process and how is it implemented?

3. Any resources you could point me to in learning about this process would also be appreciated.

Thank You
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grbladesConnect With a Mentor Commented:
1&2) This functionality is only on the Cisco PIX firewalls and it basically inspects the SIP and RTP traffic and alters the contents of the packets as it goes through the firewall much the same way and inteligent NAT routers do with ftp traffic.

You can have a STUN (proxy) server outside your network and configure the phones etc... to use it. A STUN server detects the presence of NAT and tries to work around the problems caused.

eidebaillyAuthor Commented:
I was just told by our line-carrier that they currently use 871 routers on up that have at least version 12.4 IOS. They say that this IOS handles the NAT transversal for VoIP. Does that sound right? So that means I wouldn't need anything but one of these routers with IOS to handle VoIP and NAT problem?
I dont know of any specif router feature to enable SIP inspection like the PIX does.
Could you ask your carrier what the command is to enable the feature on the latter IOS version?
eidebaillyAuthor Commented:
It didn't look like there was a command to enable it. It came with it. ALG: Application Layer Gateway Feature. Which does NAT transveral for VOIP. It looked like it came out around IOS 12.2 but my carrier doesn't use anything lower than 12.4.
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