In this case, it is necessary to determine a number of important points indicated below:

you need to determine which method of connecting intercoms is available at the entrance: coordinate or digital;
you need to find out from the seller as many features of the purchased model as possible; ask not only the manufacturer, but also the brand of the selected equipment;
be sure to check whether it will be possible to connect the purchased device at the entrance, where other intercoms are already installed;
based on the available funds for the purchase, the choice of an intercom with or without a display is made.
If we consider modern intercoms designed for apartments, then they can name some useful functions:

for owners of a large apartment, it is recommended to purchase an intercom without a handset. It will be inconvenient to use it. The best option is with a touch screen, as well as a remote control;
if the entrance to the house is a walk-through, you need to buy an intercom, which has two blocks. In this case, the doors can be opened to guests not only through the central door, but also through the “back door” - the back door;
another intercom function in large apartments is the ability to connect the device to a TV. The fact is that the front door can be far from other rooms. Therefore, in order not to go to the door, you can see who has come through the TV;
modern intercoms have several cameras, as well as memory cards for recording. This greatly helps to determine who comes during the absence of the owner.

I think that you should read about the best intercom systems such as videx intercoms and it will be useful for you