In a discussion around VoIP someone recently said to me that he thinks that "non-network operator supplied" VoIP is having a big impact on voice prices, both in fixed and mobile networks. I am not quite sure this is yet the case, however. When looking at prices for fixed and mobile voice calls, it can be observed in most countries where telecommunication was liberalized a decade or so ago, that prices are falling. This has consistently been happening over that time, long before VoIP came up.
From my point of view, falling fixed and mobile voice prices are more a result of competition between incumbent and startup telecom companies, who are offering voice services over circuit switched technology.
Technology has advanced, so in many cases, voice calls are transmitted over IP in the backbone networks of network operators and also internationally, but that's still virtual circuit switching and not 'end to end' VoIP over the Internet. Sure, there are services such as Skype, which are free while the call stays between two users of the same service but all people I know use it as additionally to circuit switched services rather than a replacement.
So why are VoIP services still lacking popularity today? I think it's quality of service and ease of use, in which end-to-end VoIP is still very much behind traditional circuit switched voice. That doesn't mean VoIP isn't catching up, but in my opinion we are not yet at a point where VoIP is a serious threat to circuit switched fixed and wireless voice.
As always, comments are welcome.