Hi there;
[!] Unfortunately, I couldn't recommend the installation of a PABX in this day and age- they are about to be made redundant by telstra due to the NBN, as they wont install new ISDN lines anymore.
There are much cheaper to run, more modern systems i can vouch for, which will cost you less than $15 a seat for the equipment, and you can obtain unlimited national and mobile calls with the exchange plan. call me on [!] to have a chat about smarter procurement for your business, and ill refer you to the right product :)