is your IP for your Nettalk Server bound to ip (NIC) on the Internet side? (not just the LAN ip)
i coexist with IIS, in some environments and not in others, to date i have not needed to investigate why, i have been able to simply disable iis as it was just something that MS installed by default. so perhaps its not ISA/routing, but something already using your address/port. (Stopping IIS can quickly rule it out)