I have a machine connected to the local network via eth0 as 192.168.1.39. This machine also has an ISDN connection for dialling remote machines. The ISDN connection uses RawIP, and assigns a static IP to the dialled machine (192.168.1.5)
I'm wondering how to go about making the dialled machine visible on the internal network.
The routing I have tried is as follows:

On 192.168.1.39
host 192.168.1.5 -> isdn0
network 192.168.1.0 -> eth0
default via 192.168.1.1 (GW) -> eth0

I have two gateways set up on the other machines on the internal net. 192.168.1.1 (DSL router) and 192.168.1.39 (ISDN machine)

This, however, doesn't allow me to see the 192.168.1.5 machine from anything other than 192.168.1.39.

Any suggestions?

You need to make your server a gateway. By default it won't route other servers, just because you have multiple interfaces set up.

Try
echo 1 > /proc/sys/net/ipv4/ip_forward

Iain.

thanks Iain, I already have IP forwarding set on.
Also, should I have a different IP address for each interface (eth & isdn).