I've been using this for some time now and while I only need SIP occasionally, every time I start Telephone, I'm pleased with its no-bullshit design (and the fact that it just works). This exactly how much UI a SIP client needs.
I'm using Jitsi for ZRTP voice/video calls, but only because there isn't a better client. It is sadly not behaving very well on OS X (and according to the other end, not on Windows either).