Both Intercoms and telephone entry systems are a means to communicate with someone inside a protected area. The caller initiates the call via an exterior mounted call box. The resident then has the option of releasing the door for the caller or simply hanging up.
The biggest difference between the 2 systems is the wiring. A telephone entry system requires power and a telephone line to allow it to operate. It can dial any programmed phone line, even cellular to release the door. An intercom system has wiring run to each and every location that needs to communicate to the door. A phone must be mounted inside the apartments or offices in order to talk to the visitor and to release the door.
Both telephone entry and intercom systems can be equipped with video to allow the receiver to verify the identity of a caller before releasing the door. The video from the intercom can be modulated, which allows residents to see the visitor on their TV.