Asterisk is an open source PBXi, telephony engine, and telephony applications toolkit. Available for free. URL: asterisk.org.

CentOS is a Linux distributions based on Red Hat Enterprise. URL: centos.org.

The following required before we begin installation:
[*] CentOS minimal install
[*] kernel-devel / kernel-smp-devel
[*] bison
[*] openssl-devel

Installation:

We will use atrpms repo for this install.

Login via SSH using root to the server.

Create the following file:
/etc/yum.repos.d/atrpms.repo

Put the following in it:
[atrpms]
name=CentOS $releasever - $basearch - ATrpms
baseurl=http://dl.atrpms.net/el$releasever-$basearch/atrpms/stable
gpgkey=http://atrpms.net/RPM-GPG-KEY.atrpms
gpgcheck=1

Or you can download the file [url=/files/page/atrpms.repo]here[/url] and save it as /etc/yum.repos.d/atrpms.repo.

We need to update the server, do:
yum -y upgrade

If that does not work for you, try:
yum -y update

Repeat the update until no more update left.

Install asterisk:
yum install asterisk

You will get something like:
Loading "installonlyn" plugin
Setting up Install Process
Setting up repositories
Reading repository metadata in from local files
Parsing package install arguments
Resolving Dependencies
--> Populating transaction set with selected packages. Please wait.
---> Package asterisk.i386 1:1.4.9-43.el5 set to be updated
--> Running transaction check
...
...
...
Installing: asterisk ######################### [6/6]

Installed: asterisk.i386 1:1.4.9-43.el5
Complete!

Note: do not rely on above output, I’ve cut some part of it. Most likely your output will be different than mine.

You will need to configure a few things:
nano /etc/asterisk/sip.conf
nano /etc/asterisk/extensions.conf

We are done.

Start asterisk:
/etc/init.d/asterisk start

Good luck!