I recently came across a press announcement that a German mobile network operator and a security company providing hardware and software for encrypted mobile voice calling have teamed up to launch a mass market secure and encrypted voice call service. That sounds cool at first but it won't work in practice and they even admitted it right in the press announcement when they said in a somewhat cloudy sentence that the government would still be able to tap in on 'relevant information'. I guess that disclaimer was required as laws of most countries are quite clear that telecommunication providers are required to enable lawful interception of the call content and metadata in real time. In the US, for example, this is required by the CALEA wire tapping act. In other words, network operators are legally unable to offer secure end-to-end encrypted voice telephony services. Period.
Wire tapping to get the bad guys once sounded like a good idea and still is. But from a trust perspective, the ongoing spying scandal shows more than clearly that anything less than end-to-end encryption is prone to interception at some point in the transmission chain by legal and illegal entities. Also, when you think about it from an Internet perspective, CALEA and similar other laws elsewhere are nothing less than if countries required web companies to provide a backdoor to their SSL certificates so they can intercept HTTPS secured traffic. I guess law makers can be glad they came up with CALEA and similar laws elsewhere before the age of the Internet. I wonder if this would still fly today?
Anyway, this means that to be really secure against wire tapping, a call needs to be encrypted end-to-end. As telecom companies are not allowed to offer such a service, it needs to come from elsewhere. Also, this means that there can't be a centralized hub for such a service, it needs to be peer-to-peer without a centralized infrastructure and the code must be open source so a code audit can ensure there are no backdoors. Also, no company can offer the service as it would be pressured and probably required by law to put in a backdoor (see e.g. Lavabit and Silent Circle).
This is quite a challenge and requires a complete rethinking of how to communicate over the Internet in the future at least for those who want privacy for their calls. And without companies being able to provide such a service it's going to be an order of magnitude more difficult. For private individuals this probably means that they have to put a server at home for call establishment and to tunnel the voice stream between fixed and mobile devices behind firewalls and NATs. For companies it means they have to put a server on their premises and equip their employees with secure voice call apps that contact their own server rather than that of a service provider.
While I already do this for instant messaging between members of my household (see my article on Prosody on a Raspberty Pi) I still haven't found something that could rival Skype in terms of ease of use, stability and voice + video quality.