FreePBX allows the creation of six different types of trunks as follows:
Zap, IAX2, and SIP trunks utilize the technologies of their namesake. These trunks have the same highlights and pitfalls that extensions and devices using the same technology do. Zap trunks require physical hardware cards for incoming lines to plug into. SIP trunks are the most widely adopted and compatible, but have difficulties traversing firewalls. IAX2 trunks are able to traverse most firewalls easily, but are limited to adoption mainly on Asterisk-based systems.
In terms of VoIP, ENUM (E.164 NUmber Mapping) is a method for unifying E.164 (the international telecommunication numbering plan) with VoIP routing. The ENUM system can be considered very similar to the way that the Internet DNS system works. In the DNS system, when a domain name is looked up an IP address is returned. The IP address allows a PC to traverse the Internet and find the server that belongs...