Invisible Multi-Target Doorbell

Several companies sharing the same building all need a doorbell for visitors. The exterior of the buidling may not be altered significantly. Do you have a solution?

We hereby ask Elektor Labs users to propose ideas, suggestions and possible solutions for a multi-target doorbell (see below). This is not a contest (although we may reward the best solution), just a request for your participation. If a good, practical & original solution comes out of this, it will be implemented.

The Problem

A building in the old city centre of Amsterdam houses several companies, Elektor being one of them. To signal their presence, visitors must be able to reach each of these companies through a single, multi-target doorbell. An old-fashioned metal pull lever bell will be added soon. The exterior of the building may not be altered significantly; cabinets full of high-tech electronics may be placed in the space behind the front door (as long as people and objects can pass comfortably).

Update

Following the discussions that have developped in the comments below we have renegotiated the specifications with the owner of the building:

it is allowed to drill holes for cameras, but not in the door;
it is not allowed to build a display in the door, however, a display in the doorpost is possible;
it is allowed to mount a stylish panel on the building (or place it behind a window, see project photo above). Such a panel could house a display, but not a pushbutton per tenant. Thus a touch display would an option.

Currently there are two doorbell panels considered ugly and that must be replaced or removed. These panels measure each 40 x 48 x 4 cm (w x h x d).

47 knowledge mile actual doorbell — 47 Knowledge Mile actual doorbell

What we don't want

We DON'T want this, we are looking for an invisible solution.

Let it be clear that the problem we are trying to solve is not distributing the bell signals inside the buiding (we know about multiplexing, busses and all that), but getting the bell signal from the street side into the building in an elegant, high-tech and discreet way.

Specifications

Each company (doorbell target) must be reachable individually without notifying other companies (targets);
One generic bell, call or code is required for visitors who do not know their target;
The exterior of the building may not be altered significantly;
Keep the current doorbell;
New doorbell on the street side should be as discreet as possible: small, invisible;
Accessible to disabled people;
Respect privacy regulations;
High-tech gadget solution is greatly appreciated.

Some possible avenues to explore

Keep in mind that people with an appointment can be treated differently than people without. The first group can get a code or password, or provide a photo before they show up at the front door.