I myself don't have a fixed way of saying it. So many people in the world pronounce it in so many different ways. The most commong pronunciations are:
vee-oh-eye-pee (It's like spelling it)
voyp (like in voice or void)
Voice over IP (that's rather a phrase)
internet telephony (maybe that's... evading acronyms)
There is no pronunciation that it more correct than the other, since English is a dynamic language to which certain words are added or changed on popular demand. I'm not sure whether VoIP has entered the dictionary yet, though.
'Voyp' (like in voice) and V.O.I.P (vee-oh-eye-pee like spelling it) are the most common pronunciations, but most of those who are not used to the technical term and who know the technology merely as one allowing to make phone calls over the Internet call it either 'Internet Telephony' or 'Voice over the Internet'.
I myself am more at ease with vee-oh-eye-pee, like I say ess-kew-el (geek people normally say see-quel for SQL). I like it maybe because it is more faithful to what the letters represent. But at times of quick blabbering, I realize the importance of 'voyp' and 'see-quel'. So, for me, 'voyp' is quick and slang type and 'vee-oh-eye-pee' is more like 'clean' and pretentious language. But that's only very subjective.
I feel that when an acronym gets a continuous pronunciation like a word ('voyp'), it is a sign of popularity and success.
Did you know the following are pronounced thus?
EBCDIC - eb-cee-dik
WSDL - wiz-dull
WYSIWYG - wi-zee-wig
GUI - goo-ee
ASCII - ass-kee
So, be at ease with 'voyp'. Whichever way you say it, there still will be voice and IP in it!
The small 'o'
Spelling VoIP is no big deal as it is an acronym of four letters. The only thing is that while the letters are in capital (like in all acronyms), the 'o' for 'over' is written in lower case. This is because it is a preposition, and you know how acronyms treat prepositions.
Back to pronunciation: my other daily language is French, so now I have to sort out how to say VoIP in French... whether 'vé-oh-ee-pé' or 'vwap'!
