When you think about VoIP networking the first thing is to make sure your cabling will support it, must be CAT5 or higher and I would have it certified to verify that all connections are good.
Also, its a good practice to separate your voice traffic from your data traffic through the use of VLANs. Another thing that you will want to consider is that if you use Cisco phones you will probably want to use switches that are power over ethernet (PoE) capable as the phones will pull power fromt the switches. Designing a VoIP network is no small task by any means, if funds are available I would bring someone in to help design it. Hope this helps, there's just so much involved I just went over a few of the highlights.