Voice over Internet Protocol. A technology for transmitting ordinary telephone calls over the Internet using packetlinked routes. Also called IP telephony
There are two major reasons to use VOIP
- Lower Cost
- Increased functionality
A way is required to turn analog phone signals into digital signals that can be sent over the Internet.
This function can either be included into the phone itself or in a separate box like an ATA .
ENUM - The bridge between the switched telephony network and the Internet
ENUM RFC 3761 is a protocol the uses the Internet DNS system to translate E.164 (i.e. ordinary) telephone numbers into IP addressing schemes (like SIP, H323 or Email). RFC 3761 has replaced the former RFC 2916 in May 2004.
Current ENUM deployment status in various countries
RFC 2870 : Root Name Server Operational Requirements
ENUM is already supported by SIP Proxies like SER or SNOM 4S, VoIP gateways like Asterisk , Cisco) and SIP phones (SNOM 200).
From the RFC abstract:
This document discusses the use of the Domain Name System (DNS) for storage of E.164 numbers. More specifically, how DNS can be used for identifying available services connected to one E.164 number.
Routing of the actual connection using the service selected using these methods is not discussed.
In short, a server with ENUM support will lookup a dialled telephone number in DNS to see if there's alternate ways to set up the call instead of just calling out on the PSTN telephone line. ENUM may contain a reference to a SIP URL ? , a telephone number to dial, a web page or an e-mail address.
ENUM 
