What is ENUM and why whould I want to set it up in Asterisk?

Simply put ENUM is a way for those of us that utilize VOIP to avoid using the PSTN to establish a call between to VOIP systems with out using the PSTN.  Think of it as a true Internet Phone - no phone company of any kind needed.  Your probably asking "Don't I do this already?".  Well if both you and the person your calling are using the same VOIP provider you are doing the next closest thing but still using a third party to complete your call.   Imagine being able to call from one * box to another * box directly over the internet without involving a third party.  ENUM offers us this ability. 

UPDATED: Click here for an ENUM HOWTO for Trixbox 

Click here for a HOWTO for inbound calls via ENUM and SIP on AAH

Click here for a HOWTO for inbound calls via ENUM and IAX2 on AAH

 Your phone number and server information are stored in a special DNS zone (very similar to a reverse lookup zone).  When an ENUM enabled * box calls your phone number, it will first attempt to find your phone number in the ENUM DNS zone.  If it finds your phone number it will use the infomation to setup a call directly between their * box and yours.   Here is a sample call flow:

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 Your * box answers and forwards the call to your extension

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You answer the phone

Think about the potential...  you can contact someone via VOIP with no prior knowledge or configuration. Remeber, ENUM has no effect on how traditional PSTN users or VOIP users who are not using ENUM contact you.  ENUM only adds another option for other ENUM connected VOIP users.  

The benefits of ENUM really ring true when working with a per minute VOIP provider or when you make a lot of International calls.