[VoiceOps] Cisco 7941 SIP

Pete E peeip989 at gmail.com
Tue Oct 6 17:09:59 EDT 2015

Greetings Voice Operators,

We have an interesting (code word for annoying) challenge that we've never
dealt with before, probably because we don't do much with Cisco phones. We
have a new customer coming on who wants to keep their very old Cisco 7941
phones. They have a few offices and the phones work as expected behind an
Edgemarc. However, they also have 100+ home users, and that's where the
issue comes in.

Apparently Cisco introduced a security "feature" where they create the
session using a random high numbered port (e.g. 49123) but in the Via
header, they say to respond to *private IP, port 5060*. So when the SBC
sees the private address it assumes it is being NAT'd through a firewall
and replies back to *public IP, port 49123*. What we're seeing is that the
home router passes the response back to *private IP, port 49123*, which the
phone doesn't accept (because it wants it on 5060) and the REGISTER fails.

As you know most home routers are poor at handling ALG (and we've tested
and found they are equally bad at handling this scenario). We (and the
customer) don't want to troubleshoot 100+ individual home routers.

We haven't found a way to turn off this really awesome "feature" so we're
trying to find other solutions. Anyone been through this and have any

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