First instal vpnc_
sudo apt-get install vpnc
Now, find the .pcf file that your office provides you. Open it in a text editor and use the relevant info to create file /etc/vpnc/default.conf. The contents shoudl look something like this_
IPSec gateway 123.456.789.123
There's one hold up, though, as many companies provide their secret phrase pre-hashed. This means that it can't be entered correctly into the configuration file for vpnc.
(your pre-hashed secret string will look something like this)
The solution is to hop over to this cisco vpnclient password decoder, pop in your hash, and retrieve the key. If you're really paranoid, you can download the C source code and run it on your machine.
Finish filling out the conf file, save it, then run
sudo vpnc --natt-mode cisco-udp /etc/vpnc/default.conf
To disconnect, run