Here's what we did to build an Asterisk server under FreeBSD-5.2.1.
The same approach can be used for any software in the 'ports' collection.
We needed to update the ports collection to the latest version so that
it built the latest version of Asterisk.

1. Set up cvsup

# cd /root
# cp /usr/share/examples/cvsup/ports-supfile .
# vi ports-supfile

Change the 'default host=' setting to point to a cvsup mirror from the list
at http://www.freebsd.org/handbook/cvsup.html#CVSUP-MIRRORS

2. Run cvsup

# cvsup -g -L 2 ports-supfile

The ports collection is now up-to-date. If you already had the ports
collection installed then this is a very efficient process, because it
only transfers the differences between the files you already have and
the latest versions.

3. Build the 'portupgrade' utility, which knows how to build ports and
update their dependencies

# cd /usr/ports/sysutils/portupgrade
# make all install

(Incidentally, you can now type 'portversion' to get a list of all packages
installed on your system and whether new versions are available in the ports
tree. A useful command is 'portupgrade' which will rebuild packages to their
latest versions)

4. Build asterisk (automatically installs other ports required, or upgrades
older versions of ports it needs which you may already have installed)

# portinstall asterisk

If this aborts, you fix the problem then do 'portinstall asterisk' again
(it will continue more or less where it left off).

# vi /usr/ports/devel/pwlib/Makefile
comment out the FORBIDDEN line

# vi /usr/ports/net/openh323/Makefile
comment out the FORBIDDEN line

If you get an error about being unable to upgrade an older package version,
try

# FORCE_PKG_REGISTER=yes portinstall asterisk