How do you set up a telephone system in an office?

Depending upon the size of the operation and the physical layout of the rooms you will be networking, companies do sell small business systems that you can install yourself. I worked for AT &T Home and Small Business Division, before they evolved into Lucent Technologies, and we sold several systems that could handle up to 4 telephone numbers, transfer calls between phones, provide intercom abilities, etc. Check the included websites to see if they will provide the capacity that you require.