Voice is an Application, but Telephony is a Service
I realize that by using the word "telephony" in the title, I've immediately categorized myself as a bellhead. A curmudgeon. An anachronism.
But it struck me the other day, when seeing an ad for one of the many cable voice services for only $30/month, or $35/month, or whatever - why are people willing to pay $30/month, or $35/month, or whatever for telephone service?
Because it works. Because it's easy to set up and use. Because it does what you need, and very little that you don't need. Because you don't have to configure it, administer it, upgrade it, manage it, or figure out how to make it work with anything else.
Because all that stuff is taken care of behind the scenes. The service providers provide a service - which includes configuration, management, upgrades, interworking, reliability, quality, and all the other "non-functional" requirements that you need for something to just work.
The "functional requirements" - codec, transmission, addressing, directories, and the like - can be met by an application. But ask any systems engineer - the functional requirements are the easy part.
Martin has "gone cold" on VoIP - because, basically, it's inconsistent and it's a pain. I'd posit, though, that it's not VoIP per se he's gone cold on - it's that he's trying to use an application where he's accustomed to using a service. And where he needs a service. And, therefore, where he's willing to pay for a service.
There is certainly a place for "VoIP applications". Just as there's a place for accounting applications, architectural design applications, and financial planning applications. But just as accounting applications, architectural design applications, and financial planning applications haven't resulted in massive unemployment for accountants, architects, or financial planners, the existence of VoIP applications doesn't mean there's no longer a need - or a willingness to pay - for voice services.
The key is to focus not on the "VoIP" part - but on the "service" part.