Hello,
I am new to this website, I have just started to use freelancer.com, this is why I don't have any evaluations yet..I am 34 years old, Belgian, I have more than 15 years of working experience with linux and windows.
I have already installed quite a few professional asterisk PBX's, in the beginning I was compiling everything by hand on gentoo but it is too time consuming and you can't have good interfaces for the users, so now I mostly use elastix, which integrates everything with nice interfaces and is easy to update, manage etc.. it is also very easy to find manuals for basic and advanced users.
Everything you mention is easily implemented in 3 to 5 days, and after that you can manage everything by yourself, I would show you or your technician(s) how everything is done. Of course I would stay available for further assistance.
It is possible to circumvent any ISP VoIP limitation using IP tunneling (VPN), as for your laptop, I would use Counterpath's Bria software.
Thank you, regards.
Update:
1) Of course! But do you already have a system installed?
2) Backup is integrated in elastix, I would also install webmin which includes another backup system. Dropbox volume can be mounted on linux, so no problem here
3) Just a few clicks..but you need space if you have lots of calls
4) That's the point of a PBX..
5) Windows clients can connect using openvpn yes.
6) Lot of documentation already available for elastix, but as I said, I would show everything to your technicians.