iptables forwarding question

[James Pifer]

Is it possible to forward all traffic, not just a port, that comes from
a specific ip address to another address? Let's say you're running
iptables on a machine that is acting as a router on an internal network,
so traffic by port is not blocked. 

I've been searching for rules and everything I'm finding is forwarding a
specific port. Is this possible? If so, an example rule(s) would be
appreciated.

If interested, more in depth explanation below. 

Thanks,
James

I'm using pptpclient to connect to another network using LAN to LAN.
After the connection I have eth0 and ppp0. pptpclient automatically sets
up iptables rules for the routes and masquarade. This pc is also setup
to act as a router. I'm trying to get an ipphone working over this
connection.

I found a firmware upgrade for the phone that updates it to
support NAT. That was a huge piece of the puzzle, but I think I still
have a NAT issue. For background here's how I understand the phone to
work in our environment.

The phone does registration and call setup with server1.
Once a call is placed, or the line is picked up (to hear dial tone for
example), it's turned over to server2.

In my case all communication with server1 works great. My phone gets the
time, it rings if someone calls me, and I can even ring someone else,
although I hear nothing.

Using ethereal I can see the packets coming back from server2 and
hitting ppp0, but it's like my machine doesn't know what to do with the
packets. I'm thinking maybe when the call is turned over to server2,
that it's not correctly sending the packets with NAT in mind? I don't
know NAT to that depth so it's a shot in the dark.

So I know the packets coming from server2 are destined for the phone, so
I was hoping I could just try routing/forwarding them to the phone's ip
address.