A Windows Firewall has very important role in VOIP telephony. It should make a job harder for the attacker who wants to compromise particular Windows machine on the network hence an IP telephone connected directly to that computer. Some companies have an external firewalls which protects all devices within the company although that solution might be not sufficient for the entities that should have own protection switched on.
Wherever your business has been equipped with well protected devices, router firewalls, an internet security software etc… there is always good to make sure that each Windows machine has own protection updated and running.
