Good question,
1. You don’t have to use wireless..
There are many ways, such as laser, ordinary cabling is enough…(for transmission medium)
For transmission method, you have no need to use VoIP, if it takes lot of money…You have to study your telecommunication cost before
2. The equipments are a lot…
Do you have PABX ? If yes, you need signalling gateway, for better quality: voice gatekeeper maybe needed, etc…learn about it much from http://www.iptelephony.org or http://www.cisco.com