Ward’s advice really rings true (sorry for the telecom geek pun, it couldn’t be helped!) His “Baker’s Dozen SIP Security Checklist” makes perfect sense. That doesn’t mean that I can’t add my own two cents.
ISNs are a novel approach to bridging the gulf between the worlds of legacy phones with traditional phone numbers, and the realm of SIP URIs. It’s a way of dialing something like a normal number and having it automatically be resolved to a SIP URI. This is easily implemented using Asterisk, especially when Nerd Uno shows you how.
All of this is an approach to transitioning from our current state into a time when SIP URIs are more commonplace. That would imply that more of what we think of as “phone calls” would transit a pure IP network. That fact is a major enabling mechanism for such improvements as wideband audio & improved call security, amongst many other things.