Many managers find this to be the best way to quickly monitor activity within the office or department. Find out if your telephone system has any limitations in terms of how many people' s calls can be covered by another telephone, and how many different telephones can cover for a particular extension or direct-dial telephone number. If covering means anything but a discrete specific button on the telephone associated with each extension or direct-dial number to be covered, be skeptical. See what programming is involved. If the answer is not clear to you, it may not be to others who are using the system.
Most displays are indeed hard to see.
Lucent Technologies once had a beautiful display with characters in a readable bright blue, but it was discontinued. Many display messages are written in cryptic telephonese such as TK 001 or XFER VM, which means little to the user. Spend some time setting up the messages that appear on your displays, making them meaningful and easy to understand, and you will automatically improve call coverage. A good application for displays is caller ID, which lets you to see who is calling from the outside.