From Guardian Project Wiki
What is Twinkle
Twinkle is a desktop VoIP client which has had ZRTP support since, like, forever. Unfortunately, the UI sucks (to borrow a technical term from the late Steve Jobs). It only runs on Linux. It was used by OSTN as one of the first compatible clients and offered good security compliance.
Twinkle does not support SIP over TLS, and fails silently. We are working to fix it so that Twinkle supports Ostel.