When you are away from home, you can think of cheaper and reliable alternatives to call home.
Diamond card, you can use twinkle, QuteCom or Ekiga as your SIP client /soft phone software in ubuntu. I found twinkle better in ubuntu 9.10
Diamond card is a SIP based VOIP provider with cheaper rates. I can call india at INR 1.21, this is kind of very cheap or can compare with STD charge in some parts of India. Now a days concept of local and STD calls is disappearing with a country wide local rates.
Busta, I think it comes with its own client which is available for windows platform. I haven't found a linux based client for busta.
Skype, yahoo etc are the default soft phone solutions general users can think of, but their tariff seems to be on the higher end. skype has ubuntu version, I do not think yahoo is available as softphone in ubuntu.
There are lot of other alternatives available on internet, if we type SIP and cheap internet calls, we are so many alternatives.
http://www.wengophone.com seems to be one platform indepent client, similarly X-lite comes in support for multiple platforms.
Reliance I call is providing the cheapest calls on internet (so far calling indian numbers are concerned). I am yet to try the SIP tweak (using some info given on another blog). Reliance I Call formally supports windows boxes only.
If you are reading this post and know better alternatives, please post a comment.