Lookin in synaptic (the GUI package manager for Ubuntu) there seem to be a few possibilities - kphone, twinkle, etc. - any recommendations for what works well etc. ?
I've been using Twinkle with Gradwell VOIP for a good year and more, having done the rounds of the applications you mention along with x-lite, and a few others - it was as easy to set up as any of them, and it works well.