Hi,
I have been working with Cisco VoIP since 2001 and Asterisk and freepbx ,trixbox, elastix since 2007.
I would recommend using Asterisk and freepbx , I have installed this kind of setup quite a few times in Australia as well,
you just need to give access to the VM or the physical machine and a list of names and phones,
also the account settings are needed from Engin.
That being said I would like to have a little chat about your project to better understand the needs of the project
regards,