I must admit that I don't know exactly what Twinkle does in detail. If you insist on using SIP, you might have a look at Ekiga, AFAIK it can be used as a SIP softphone.
However, my preferred VoIP solution these days is Jabber (XMPP), and Empathy as a client on Linux systems. It even supports video chat.

It works very well “out-of-the-box” i.e. after emerge – given of course that your webcam and sound in/out already work. If your conversation partners are on Windows, they might try Pidgin for XMMP voice and video.