Speedtouch 510v4 on both ends & "not responding"

Question

Both with latest firmware 4.3.2.6
Both with default settings, UPnP is on and SIP 5060 is unbound on both.
We have someone on dial-up and we can successfully call that person and be called by the person, but when we call each other it's always "not responding" or "other side cancelled".
My Speedtouch is connected to an Airport Express which is acting as an access point only which doesn't distribute IP address. The other Speedtouch is also connected to an Access Point which is an SMC. So both Macs are getting their IPs directly from the Speedtouch.

Answer 1

Yes it should work but there are external issues outside of your control at the moment - Apple/AOL servers are themselves displaying unwanted behaviour, particularly if you're trying to chat using an AIM account with dotMac (synchronisation issue).
Once you've downgraded to 4.2.7 and gone through the setup wizzard for your ISP - make these changes using Terminal (Applications/Utilities):
telnet 10.0.0.138
username:
password:
system config upnp=enabled
This command will display port bindings:
nat bindlist
If you see SIP 5060 in there, remove it thus:
nat unbind application=SIP port=5060
Save all changes
saveall
Restart the router
system reboot
Further reading:
http://www.aptr36.dsl.pipex.com/index.html