Exclusive by Auerswald: the a/b switching module
If pressing the bell button is supposed to activate other functions as well, an a/b switching module pays off immediately: it takes care that e.g. the light in the hallway goes on or the existing door bell rings. The modules are simply interconnected at any point of the signal line between door intercom and the analogue (a/b) port of your telephone system.
When the bell button is pressed, a special control signal triggers the desired switching process. If e.g. you are talking to the visitor at your front door, you can release the door opener by pressing a key on your phone. And if you install several a/b switching modules, you can control various switching processes with different frequencies.
Communication and switching using the mobile phone
Assuming you are out of the house for an errand. If the door call is forwarded to your mobile phone you can inform the person in front of the door that you will be back in a second. Or you can tell the postman where to leave the parcel. If it is a family member who has forgotten the key you can open the door with your mobile phone, switch on the light or activate other switching tasks via the connected a/b Switching Modules.