Most of the VoIP discussions and tips have revolved around the challenges of providing minimal impact to the users' experience with similar features and 'look and feel' and of course, just getting the stuff to work; cohabitating inside your own network with other LAN and WAN traffic. But as VoIP matures, we're getting past the problems in our little islands, and starting to look toward the inevitable day when we connect to other organizations via VoIP instead of VoIP-to-TDM/PSTN-to-VoIP.
While we've seen a lot of residential VoIP service providers and are beginning to see a lot of small-to-medium business VoIP service providers, you don't have to use one of these to start preparing yourself for the future. In fact you can start today, for free, with no impact to your users.
As a network administrator, you owe it to yourself to get ahead of this curve: go learn about ENUM. I'll loosely compare this to your ability to register Domain Names without actually hosting a website. You can learn about the technology and start planning how you'll use it in your organization, and personally, without actually having a monthly VoIP telephone bill.
For the hard-core, go read the following RFCs:
2915: NAPTR RR
2916: ENUM Protocol
3761: ENUM Protocol (this obsoletes 2916)
3762: Usage of H323 addresses in ENUM
3764: Usage of SIP addresses in ENUM
For some lighter reading, visit the ENUM forum.
When you're ready to start playing with registration, go to e164.org or try enum2.go.
This protocol is important because, much like DNS, it binds together so many other technologies. Understanding the capabilities we expect in the future from this protocol will help guide you as you design related systems over the next few years.