There are several options. One you could look at is twinkle.
http://www.xs4all.nl/~mfnboer/twinkle/
This one seems to have a good reputation
(but there are others as well: linphone, ekiga and several others
whose name escapes me atm)
FM
I use openembedded. See
http://www.elinux.org/BeagleBoard#Development_environments for
pointers on how to get started.
If you have no experience in this maybe look to see if there is
another sip client already available (I think ekiga is).
You might also compile natively. I'm not 100% sure if there is a
package build-essential, but if not it is definitely possilbe to
install the individual tools like gcc and make.
Have fun, Frans
PS: I'll put twinkle on my todo list, but don't hold your breath, that
list is about to overflow.