If i need a network operating system with the following requirement, which show i use?
-MNC
-Printer Sharing
-connection must be stable
-Connection can support high traffic
- The company used voice over IP for their telephone system (Not sure about this)
-The company is adding more office in another country after the setting up of the NOS so it will be good if its not that complicated to add connection to the oversea new branch in future.
It will be good if i can get some idea why your choice is over the others but having the recommendation is good too!
Thanks. 
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I really don't know what an MNC stands for, but if you want a stable operating system that can do all these things you have a choice of a few operating systems.
It does sound like you need to consider whether a directory would help.
Therefore, Windows Server 2008 can definitely do all these things, with the right hardware. For VoIP, you'd probably need to consider a software product called Office Communications Server 2007. For the directory you'd use Active Directory. File and printer sharing is built in. The OS itself is very stable, just make sure you get quality hardware.
RedHat Enterprise Linux might be another operating system you'd like to consider. You can setup Samba for the file and printer sharing, Asterisk for VoIP, and they have a directory solution.
These are just two solutions. If you are setting up an expanding company, I think that Yahoo Answers is probably not the place to go, I'd try talking to an IT consultancy company.