Hi,
I have been working with Cisco VoIP since 2001 and Asterisk and freepbx ,trixbox, elastix since 2007.
I have skimmed through the document you attached and nothing seems to stand out that Asterisk server cannot do,
If you can give a list of features that you want to have that would be great.
If the project turns out to be a simple asterisk install i can even give discount on the total fees.
we can even go for 3cx ( windows based ) but that is not free and open source.
I would highly recommend to go for Asterisk if you want to move to VoIP world,
It is free, it is having a long list of features , it is programmable to work with almost any cti api, etc.
That being said I would like to have a little chat about your project to better understand the needs of the project
regards,