Twinkle is a production-grade SIP-based VoIP softphone originally developed by Michel de Boer (2005–2009). It implements a full SIP stack (RFC 3261, 3262, 3515, etc.) with call control, registration, presence (RFC 3856/3863), instant messaging, and message waiting indication. Audio is handled via multiple codecs (G.711, GSM, Speex, iLBC, G.722, G.726, G.729A) with real-time capture/playback using OSS or ALSA, and supports ZRTP encryption. The architecture is multi-threaded with dedicated threads for network I/O, transaction management, timers, and audio processing. It provides both a Qt5 GUI (with QML overlays) and a console/CLI interface. Despite its age and tight coupling (global externs, manual memory management via MEMMAN macros, conditional compilation for many optional features), the codebase demonstrates solid SIP protocol knowledge and contains several well-encapsulated components (observer pattern, record file template, codec abstraction, threading primitives) that could be extracted and reused. However, the project is effectively unmaintained (last release 1.10.0 in 2016), with heavy dependencies on outdated libraries (commoncpp, ccrtp, libzrtpcpp) and incomplete Qt3→Qt4→Qt5 migration.