Phono is a simple jQuery plugin and JavaScript library that turns any web browser into a phone; capable of making phone calls and sending instant messages. You can even connect to SIP clients; all with a simple unified API.
There are some amazing APIs popping up out there and this is an example of one that I could imagine being really handing for instant call support. The only problem which is stopping me from jumping in now is US domestic calls only at the moment. If you try it out let me know.