I'm a big proponent of VoIP, partly because I've always liked the idea of getting rid of the additional burden inside the enterprise of maintaining the traditional phone network. I also have to admit that I like the idea that it challenges the way we think about communications regulation. But as this c|net News article points out, there is a big difference between VoIP in a controlled environment and using it for general purposes telecom services on the Internet.
The problem basically comes down to standards. There are two primary standards: H.323 and SIP, but I've found that even when you can agree on a standard (as you can in a controlled environment) that gear still may not be interoperable. The only way to assure that my AudioCodes boxes will work with a VocalData softswitch is to test them. The answer is that they will if you carefully match AudioCodes boxes and versions with VocalData versions. When its time to upgrade you may need to upgrade both ends simultaneously to ensure that nothing breaks.
Imagine the problem then of just giving your customers a SoftPhone and telling them they can call people on other networks with different equipment and software. Right now, its probably the rare exception when that works. The short term answer will be to route VoIP traffic destined for another network onto the PSTN. That hardly fulfills the promise of VoIP, however. The only thing that's going to bring it about is tighter standards and better interoperability testing. I'm confident that this will happen---some mighty big players are betting on it and they're going to push the VoIP vendors hard in 2004.