Before you jump too far in the direction of IP telephony you must ask yourself where the basis for your interest in pursuing that technology comes from? If your real proclivities and interests match what suggestions you've been given, go ahead and pursue this subject matter - indeed, it's a big growth area and will only continue to grow and expand for the foreseeable future. But I'm a great believer in letting your interests and inclinations play an important role in selecting technical career choices, because genuine internal interest in the subject matter will keep you engaged and involved in your day-to-day work routine long after the thrill of a new job or a bigger paycheck fade into oblivion.
But enough of this philosophical advice, except to point out that unless you've been playing with VoIP or have been exposed to it somehow already, you'll be jumping into a fairly challenging technical arena stone cold. Thus, I have to ask (but of course can't answer): why not return to your previous experience base in LAN/WAN technologies?
As far as voice over IP/IP telephony certs go, there are in fact some good vendor-neutral jumping off points you can look into, though digging into the Cisco stuff will also cover much of the same ground (and in addition to the various IP telephony specialist credentials, you should also consider the CCVP and recognize that there's even a voice track now available for the CCIE these days, too). The TIA, or Telecommunications Industry Association, offers two worthwhile certifications that focus on how telephony intersects with IT. They offer Certified in Convergent Network Technologies (CCNT) credential, and a Convergence Technologies Professional (CTP) certificate programs, both of which are worth looking into.
Beyond these offerings, you'll also find that the National Association of Communication System Engineers (NACSE) also offers numerous vendor-neutral (but tightly focused) credentials on telecom topics as well, but none focus exclusively on IP telephony.
Finally, in addition to Cisco, Avaya, Lucent and Nortel all offer certifications that also deal with IP telephony and/or VoIP topics.
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