[cisco-voip] Is it possible to disable the SIP nat helper?

[John Lange]

The Cisco IOS devices have a built in NAT translator which automatically
recognizes certain protocols and does some deep packet manipulation on
them.

For example, SIP packets have their source IP and ports manipulated to
match the routers external IP and a random port.

For some reason, this seems to be causing havoc with our Aastra phones.
The phones receive calls just fine but outbound calls fail. Packet
traces show that the server is trying to authenticate the phones but
it's failing.

The strangest thing is that other models of phones and soft phones work
just fine.

Id like to disable the Cisco NAT/SIP manipulation and just leave it with
doing generic UDP NAT to see if this resolves the problem (the SIP
server, Asterisk is smart and knows how to handle NAT without
manipulated packets).

Is there a way to disable the SIP/NAT filter?

I thought:

# no ip nat service sip

would be the answer but this only allows you to turn of SIP/NAT on a
non-standard port. Entered as above it gives an "incomplete command"
error.

Regards,
-- 
John Lange
http://www.johnlange.ca