Acquiring the skills to be successful with voice over IP is challenging for most people because it requires a broad knowledge-base in two domains (voice and data networking) plus a functional understanding of the protocols and hardware that connect these two areas (thus the name "convergence").
Fortunately, most people interested in this field start with a reasonably solid grasp of one or the other (voice or data networking). But many people don't know where to begin to find out what they don't know. In my experience, the best way to find out what it is that you don't know, is to pick a respectable certification and pursue it. Use it as a study guide and learning tool, with the added bonus of a couple letters to put in your email signature once you pass. Unfortunately, there isn't a very large choice of respectable VOIP certifications, but there are a few:
Start with this Voice over Internet Protocol exam. It is by far the cheapest, at $49, and it is really quite good. Since you take it on-line, it is easy to use as a learning tool. Simply do a print-screen on any questions you don't know, then go and research the answers at your leisure.
Next, whether or not you use Cisco, take the company's exams. You will learn more than you ever wanted to know. These exams aren't set up in such a way that you get a certification when you pass (although you will, if you take a few other exams in addition to these). Instead, they are set up to help you learn the material, and Cisco provides a suggested reading list for each exam. There are exams covering each topic:
CIPT for IP Telephony
CVOICE for Voice over Frame-relay, ATM and IP
DQOS for QoS
Each of these is at an intermediate level. If you really, really want to know the material at an advanced level, you should try the CCIE program's Voice qualification exam (which is due out in beta later this summer).
Finally, Avaya has a certification program that covers the voice, data and convergence fairly well. Its IP Telephony elective exam is an admirable accomplishment, but considerably less vendor-specific than any of the Cisco exams.
Thomas Alexander Lancaster IV is a consultant and author with over ten years experience in the networking industry, focused on Internet infrastructure.