As an intercom a Auerswald TFS-Universal Plus was chosen. It fits perfectly into the letter box and can be easily integrated into openHAB through switching and relay contacts.
The doorbell signal is forwarded through universal interface to openHAB and sent via push notification to my phone.
The door opener is activated by a switch actuator together with a 220V / 12V Relay..
Finally, the intercom can be called via a "5-digit PIN" protected external phone number and can be controlled from outside e.g. from the office.