If your telephone switch is going to sit in a dark room and shovel voice packets back and forth, you aren't going to be needing a keyboard, a mouse, and the latest 27" display along with an accelerated graphics processor card with 1GB of memory, an advanced pixel shader, etc., etc. In fact, you might not need to have any kind of keyboard, mouse or video display attached to it at all.
Administration of a headless machine is quite possible using telnet, SSH or maybe VNC. You'll probably need a keyboard, mouse and basic video/card plus display to set the machine up, since most install DVDs now days require all of these things but, once the OS is installed and configured, these devices can be removed. Here's a few things to consider.
You can get rid of X-Windows and the whole Gnome/Desktop thing by booting the system into Runlevel 3 instead of Runlevel 5 (the default). This can be done by hacking inittab and changing the Runlevel to 3.
/etc/inittab:
.
# id:5:initdefault:
id:3:initdefault:
If you do need to attach a display and use X-Windows, you can simply log in to the login prompt that comes up on the console and then type "startx".
If you're going to be using VNC, you should install all of the packages that are necessary for a display. This includes a desktop (i.e. Gnome or KDE), the D-Bus components and possibly SELinux (although it is not strictly required, Gnome runs like crap without it). Don't forget to install the VNC package itself.
If you're going to be using telnet (which you should probably always include anyway, just in case), you need to install the xinetd daemon as well as the telnet server. Then, you'll need to enable telnet (which comes disabled, by default, even though you installed it specifically). I know this is jumping ahead, a bit, but you can easily enable it with your favorite text editor like this:
/etc/xinetd.d/telnet
Change the "Disable Yes" parameter to "Disable No".
Then, restart xinetd:
/etc/init.d/xinetd restart
Incidentally, if you're using an older display (that doesn't have EDID) and/or X-Windows doesn't recognize the display properly, it may load the wrong display parameters, once you've completed the install, and the display will stop working (usually after you reboot from the install). You can fix this by booting a CD-only OS (such as Knoppix) and mounting the new system. Then, hack the /etc/xinetd.d/telnet file, as shown above, to enable telnet. Once that is done, you can reboot the new system and you'll be able to log in to telnet, whereupon you'll be able to repair the broken X configuration.