Small offices with only a hand full of phones and phone lines have the challenge of how much phone system will be right for them. Buy too much and you waste money, buy to little and you don't get the features that you need.
Many phone companies have basic call routers called Centrex. They are a very simplified PBX allowing their customers to have basic call routing to phones in an office without having an internal phone system. These Centrex services usually have a single recorded menu and have hard coded call routing to the phones in the office.
I have been answering questions on FixYa.com about small office phone systems. Primarily the AT&T and Panasonic 4 line phones. One of the most common issues is integrating the phone with a DSL (Digital Subscriber Line). The phone usually has an intercom problem or lit line when the line is hung up.
The 4 line phone is the workhorse for small offices around the country. This phone is inexpensive, has basic features and reliable. Small offices all over the country use these phones as the simple PBX (Personal Business eXchange) they need to run their business efficiently and inexpensively.