tavshed

Go to your src directory
cd /usr/src
Download current Asterisk 1.8
wget http://downloads.asterisk.org/pub/telephony/asterisk/asterisk-1.8-current.tar.gz
Unpack it
tar xvfz asterisk-1.8-current.tar.gz
Go down to asterisk unpacked files directory
NOTE: You need to do a ls -l to see the directory created from the tar.
This is the latest release
cd asterisk-1.8-current
Download mp3 support files
contrib/scripts/get_mp3_source.sh
Run configure
./configure
Start menu based configuration
make menuconfig
select app_mysql, app_saycountpl, cdr_mysql, format_mp3, res_config_mysql
go to Extras Sound Packages
Select EXTRAS-SOUNDS-EN-GSM
Save & Exit
Run make
make
Then run the installer
make install
Stop running Asterisk
amportal stop
And start the new Asterisk 1.8
amportal start

Thanks to the following:
http://www.freepbx.org/forum/freepbx/installation/asterisk-1-8-installation