Chapter 12. Internet Call Routing

Table of Contents

SRV Records
Accepting Calls to Your System
Modifying sip.conf
Standard dialplan
File parsing
Database lookup
Dialing SIP URIs from Asterisk
ENUM and E.164
E.164 and the ITU
Asterisk and ENUM
ISN, ITAD, and
Got ISN?
ITAD Subscriber Numbers (ISNs)
Management of Internet Numbering
IP Telephony Administrative Domains (ITADs)
Create a DNS Entry for Your ITAD
Testing Your ITAD
Using ISNs in Your Asterisk System
Receiving calls to your ITAD
Security and Identity
Toll Fraud
Spam over Internet Telephony (SPIT)
Distributed Denial of Service Attacks
Security Is an Ongoing Process

There ain’t no such thing as a free lunch (TANSTAAFL).

Robert Heinlein

One of the attractions of VoIP is the concept of avoiding the use of the PSTN altogether, and routing all calls directly between endpoints using the Internet at little or no cost. While the technology to do this has been around for some time, the reality is that most phone calls still cost money—even those that are routed across VoIP services.

From a technology standpoint, there are still many systems out there that cannot handle routing VoIP calls using anything other than a dialpad on a telephone.

From a cultural standpoint, we are still used to calling each other using a numerical string (a.k.a., a phone number). With VoIP, the concept of being able to phone somebody using name@domain (just as we do with email) makes sense, but there are a few things to consider before we can get there.

So what’s holding everything up?

[106] Seriously, get your butt over to and get your ISN today. It’s simple and free, and soon all the cool kids will have one.