Telephony seems to be going the way of email, with an increasing proportion of incoming calls being unwelcome pestering. David Cowan predicts an explosion of SPIT (Spam over Internet Telephony).
The problem (for most of us) is that as the cost of making a telephone call reduces to zero (thanks to a combination of VOIP and automated voice messages), the economic structure of communications favours the spitters. Just as already with email.
There are many possible technological barriers, but the spitters will undoubtedly find ways around them. The only real way to eliminate this growing nuisance is to change the structure.
The telecoms industry already has mechanisms, such as premium rate numbers, which allow the recipient of a call to collect a fee from the caller. (These mechanisms are available to private individuals - so if you can be bothered to set it up, you can collect a few pence every time your bank makes one of those annoying "courtesy calls". I can't remember exactly where I read this idea - possibly in Martin Geddes' excellent Telepocalypse blog.)
In a service-oriented world as well, we are going to have to think creatively about the charging structure of communications. In a message-oriented architecture, the costs could be associated with the messages. In an event-driven architecture, the costs could be associated with events. While I hope there is no immediate prospect of message spam or event spam, it is worth thinking now about creating viable and sustainable cost structures so that there is never any incentive for anyone to invent SPEW (Spam over Event-Driven Web-Services).