I’ve been using Ekiga (formerly Gnomemeeting) for several years. Among free software phones it was exceptionally stable and well usable since about its 1.0 version. But it supported only the H.323 protocol which became a big problem during the time. H.323 is usually not supported by VoIP providers, it is poorly supported by the Asterisk PBX and generally it has no future. That forced me to use the development branch of Ekiga which contains SIP support.
A development version is naturally not a stable version. Ekiga snapshots suffered from occasional freezes, automatic deregistrations and other problems. I’m tired from all the problems related to Internet telephony, so I looked for a well working free software phone with SIP or IAX support. That was not easy, most of the phones are not well usable.
I’ve recently switched to Twinkle. It is stable and provides additional nice features not present in Ekiga, such as two phone lines, better user interface, user script invocation on incoming calls or logging SIP messages. A very important advantage of Twinkle over Ekiga is that calls are much less jitter-prone when something runs on the computer. The only Twinkle drawback I’ve met so far is that it is a Qt application which means it doesn’t work very well with a systray when using Sawfish.