IAX/IAX2 Port allocation

I know nothing... Well very little

I have an old Netgear DG814 with a W98 PC on one port and an FC6/Asterisk box on
another, both of which work fine.

I have set port forwarding so that 4569 IAX2 and 5036 IAX are directed to the
Asterisk box - I *think* this is necessary to route incomg calls correctly ??
( Setting this up is a bit of a pain because the DG814 doesn't register the
change immediately but the config file confirms any changes )

Now I would like to use an IAX softphone on the W98 PC but can not get it to
register to the host site. I'm assuming this is because 4569 is directing
traffic to the Asterisk box - this may be a load of old ****

Any pointers on how these ports *should* work would be very helpful.

TIA
Andy