An Analog door phone by itself can only provide a bi-directional communication once installed. In order to provide door access an extra remote controller unit need to be installed as well as a matching door latch. Additionally to connect to your office IP network, an analog door phone requires an ATA device.
An IP door phone setup is much simpler and only requires a door latch.
Door phone - Can be either IP or Analog. An IP phone will connect directly to your office network and will receive power from it using POE technology (power over Ethernet). An analog phone will need to be connected to a power source, which can be a regular phone line.
ATA device - ATA stands for analog telephone adapter. It is a device for connecting traditional analog telephones to a voice over IP telephony network. This is only required if you have an Analog door phone.
Remote Relay Controller - A unit, like viking RC-2A, that facilitates the remote opening of a door.
Door latch - The door latch allows for a remote opening of a door. There are mechanical types and magnetic types. It need to be installed by a lock smith.
As you can see from the illustration the door phone allows visitors to communicate with different extensions in an office. The communication will be done directly (an IP phone) or will require an ATA device (analog phone). If an employee wants to let a visitor in, he can dial a code on his phone which will relay a signal to the Remote Relay Controller. The Remote Relay Controller will then relay the signal to the door latch that will open the door.
Employees can also enter a code on the door phone or use magnetic access cards (if the door phone has a proximity sensor). The door phone will send a signal to the Remote Relay Controller and onto the door latch which will allow them access.