Small businesses today have a huge advantage in terms of infrastructure costs over 10 years ago. Until recently, if a small business wanted an internal phone system, they had to buy an expensive PBX system and then pay even more so that it could be installed by qualified specialists.
Nowadays there is a better and cheaper option: a VOIP phone system offers all the advantages of a conventional telephone exchange at a fraction of the cost. If your office is already connected to Ethernet so that all computers can access the Internet, you may be able to add a VOIP phone system to only accept a monthly subscription payment. For those who want to create their own system, there is free software available that can be installed on any PC computer as a virtual switchboard.
VOIP phones can now be bought practically anywhere you can buy a computer. Setting up a system is usually as easy as disconnecting the network cable from the computer and connecting it to the VOIP phone. Then just plug the network cable that came with most phones between the phone and the computer, and you're done.