(posted at 09:19PM BST) 
What immediately comes to your mind when someone mentions VoIP (Voice over IP) to you ?
A few people say Skype, some say Windows Messenger, some may even say Teamspeak but very few will say SIP (Session Initiation Protocol) or IAX (Inter-Asterisk eXchange).
... and what most people fail to realize is that out of all the above, only SIP and IAX are fully open protocols like SMTP or HTTP.
Why is it that people seem to enjoy locking themselves into proprietary protocols and making those of us who use the truly open stuff feel like lepers ?
The magic behind SIP/IAX only comes about when you publish information about your PBX in your zone, for instance, the spilsby.net zone has the following SRV record:
;; QUESTION SECTION:
;_sip._udp.spilsby.net. IN SRV
;; ANSWER SECTION:
_sip._udp.spilsby.net. 86400 IN SRV 10 0 5060 magick.spilsby.net.
With a compatible softphone or a SIP phone, you can call any e-mail address resident at the domain and if the user has a corresponding SIP account, their phone will ring and display your own SIP URI on the Caller Display.
The eventual introduction of the UK's E.164 ENUM registry should allow any PSTN telephone number to be reachable via SIP.
My development work on Asterisk (and customer requests!) has led me to test this functionality extensively and even submit a few patches back to the main codebase in order to fix a few niggles - the greatest thing about it is the potential it has to revolutionize the telephony market around the world.
The use of e-mail has all but killed the humble letter and I believe that in the future, the use of open IP telephony standards will reduce your telephone line to a mere data circuit carrying nothing but IP traffic. | 
