Asterisk is a program that runs on Linux.. it's in the Debian and Ubuntu repositories so you can install it using aptitude or apt-get
it-etch:~# aptitude search asterisk
p asterisk - Open Source Private Branch Exchange (PBX)
v asterisk-1.4 -
p asterisk-chan-capi - Common ISDN API 2.0 implementation for Ast
p asterisk-config - Configuration files for Asterisk
p asterisk-h323 - H.323 protocol support for Asterisk
p asterisk-mp3 - MP3 format support (format_mp3) for the As
p asterisk-mysql - MySQL support for the Asterisk PBX (cdr ma
p asterisk-oh323 - H.323 protocol support for Asterisk
p asterisk-prompt-de - German voice prompts for the Asterisk PBX
p asterisk-prompt-es - Spanish prompts for the Asterisk PBX
p asterisk-prompt-fr - French voice prompts for Asterisk
p asterisk-prompt-it - Italian voice prompts for the Asterisk PBX
p asterisk-prompt-se - Swedish voice prompts for Asterisk
p asterisk-sounds-extra - Additional sound files for the Asterisk PB
p asterisk-sounds-main - Core Sound files for Asterisk (English)
p libasterisk-agi-perl - Collections of Perl modules to be used wit
p python-asterisk - Asterisk Manager API interface module for
I am unclear why you would want to install asterisk, if you don't even know what it is ...
Asterisk as a switch (PBX)
Asterisk can be configured as the core of an IP or hybrid PBX, switching calls, managing routes, enabling features, and connecting callers with the outside world over IP, analog (POTS), and digital (T1/E1) connections.
Asterisk runs on a wide variety of operating systems including Linux, Mac OS X, OpenBSD, FreeBSD and Sun Solaris and provides all of the features you would expect from a PBX including many advanced features that are often associated with high end (and high cost) proprietary PBXs. Asterisk's architecture is designed for maximum flexibility and supports Voice over IP in many protocols, and can interoperate with almost all standards-based telephony equipment using relatively inexpensive hardware.
Trying to use Wildix without Asterisk installed would be pointless, as there would be nothing for Wildix to manage.
Inexpensive FXO cards for use with Asterisk