This package is not Part of the trunk, but of a branch containing a bunch of packages by sf3978 not (yet) included in freetz.
To use them, you will have to include them in your freetz-directory an build them by yourself:
In your freetz-directory change into "make" directory and do a
svn co [noparse]http://svn.freetz.org/branches/sf3978_packages/make/sipsak[/noparse]
Then you can build it (back in freetz-dir) with "make sipsak-precompiled" and might upload it to your box..
To "fully" intregrate the package, you will have to add the entry in the Config (in freetz directory):
sed -i '/siproxd/a source make/sipsak/Config.in' make/Config.in
sed -i '/siproxd/a source make/sipsak/external.in' make/external.in
Then you should find the package under "Testing" in menuconfig.
EDIT I had to "disable" patch "003-move_get_body.patch" to compile w/o plenty of "multiple definition of `get_body'" errors:
mv make/sipsak/patches/003-move_get_body.patch make/sipsak/patches/003-move_get_body.patch.nope
(you might also just delete it ;-))