Here is my (obviously biased) view of the landscape: 1) ostel.me :: Like many of the Guardian projects, this is an experiment in combining existing OSS librari

Here is my (obviously biased) view of the landscape: 1) ostel.me :: Like many of the Guardian projects, this is an experiment in combining existing OSS libraries to create an app. In this case, CSipSimple, pjsip, and ZORG. It's basically a standard SIP/RTP VoIP client with an Android UI. That means it needs to maintain a persistent connection to a SIP server at all times, which doesn't necessarily work well with the Android process model, could drain your device's battery life, and could be flaky in scenarios where you're going in and out of coverage or switching from data to wifi. However, it is ideal for the hacker crowd that wants ultimate control, maximum configurability, and enjoys occasionally drop to the command line. You're correct that it's likely not possible to do things like certificate pinning in this context. 2) RedPhone :: While it remains to be seen whether we were correct or not, our development philosophy with RedPhone was to eschew the VoIP libraries and paradigms that were originally developed for the desktop environment in favor of OSS/Free code written from scratch for the mobile environment. Our belief is that the different network and platform characteristics of mobile devices require a mobile-oriented solution. This means that we use a lightweight mobile-oriented signaling protocol instead of SIP, push notifications instead of maintaining a persistent connection at all times, techniques for establishing low-latency routing for global calling (https://whispersystems.org/blog/low-latency-switching https://whispersystems.org/blog/low-latency-switching), a jitter buffer optimized for mobile data networks (https://whispersystems.org/blog/client-side-audio-quality https://whispersystems.org/blog/client-side-audio-quality), and your normal phone number for addressing rather than a new identifier. It also means that, yes, we can do things like certificate pinning for the signaling channel. This is all obviously oriented towards the average smartphone user, though, so it's sometimes less appealing to the hacker crowd who want to use a SIP identifier or connect through their own SIP server. 2) Silent Circle :: My sense is that Silent Circle is trying to do both. Their stack seems to be based on traditional VoIP protocols (SIP/RTP), and their server-side infrastructure appears to be a FreeSWITCH box in a single Canada datacenter (maybe with a single-DC European presence now or coming soon as well?). However, they are using those desktop protocols to try to create a packaged non-hacker-oriented experience. I'm not sure if that's possible or not, but I'm obviously biased. It does cost a non-trivial amount of money, though, and their client source isn't Free.