You should only use this method if you are unable to log into the PBX through its web-based administration interface.  For more information, read the PBX Initialization steps HERE  

You will need to connect a USB keyboard to one of the USB ports on the PBX, and an appropriate video monitor.  Depending on the model of Protel LTS PBX you have, this may require the use of a VGA connection, a DVI connection or an HDMI connection.  Check to see which video output or choice of outputs your PBX has, and use the appropriate connector to attach a monitor.  You will not need to connect a mouse.

After connecting the monitor, if you do not see anything on the display, reboot the PBX.  If you have the monitor connected properly, you will see text flashing by while the OS boots up.  When the PBX has finished booting, you will see a login prompt.   type pbxadmin for the user name and press enter. Now, enter the pbxadmin password supplied with your unit. Linux does not display your password entry, so type carefully.

DISPLAY CURRENT NETWORK INFORMATION

type ifconfig and press enter.   The current IP address for the primary NIC interface will be displayed here.

MODIFY THE NETWORK SETTINGS

type sudo nano /etc/network/interfaces   and press enter (note the spaces after sudo and nano)

This will bring up the current network configuration file, which you can edit.  Use the arrow keys to navigate on the screen.

If your PBX is set to receive an IP address dynamically and you want to assign a static address, do the following:

Locate the entry that looks like this:

change dhcp to static and add appropriate entries for address, netmask, gateway and dns, similar to this:

You can separate multiple DNS entries with a space 

After you have made your entries, double-check them to be certain they are correct.  Now, hold down the control key and type x (ctrl-x).  At the bottom of the screen you should see this dialog:

Type Y, then Enter to write the changes to the config file.  Type reboot and enter to restart the system.  Your new network settings will be in effect on reboot.

If you want to restore the network settings to DHCP, simply replace static with dhcp. You do not need to remove the additional configuration lines as they will be ignored.

IMPORTANT:  If you use the CLI to change network settings, you should duplicate those entries through the web UI as soon as you can log in that way.  Changing the Linux networking configuration does NOT change the Protel config file.  Even though your PBX server may be correctly configured on the network, there are database files within Protel that will not be correctly updated if you do not make entries through the UI.  Go to the Networking/PBX IP page through the web interface and make the appropriate entries there.  You should NOT use the CLI to change network settings unless you are unable to access the PBX through the web interface.