The issues with Microsoft ISA server 2004 and 2006
Unlike most professional firewalls, Microsoft ISA server is not able to do static port mapping. Without static port mapping it is not possible to reliably connect to a VOIP provider or remote extensions. This is not a 3CX issue but an ISA server issue. The best solution is to buy a firewall such as from Linksys, Cisco, Netgear and others. Even a $100 Linksys router can do static port mapping. You can read our Firewall Configuration guides to help you make the correct configurations.
Microsoft ISA server is known to cause issues with practically all VoIP telephony systems.