Thanks for your replies.
It is an IP based door-entry intercom system. There is an external panel outside at the door and a video unit inside the house.
It is also possible to add an IP gateway so that you can answer to the visitors calls from your smartphone, once the gateway is connected to Internet through a router.
The App installed in the smartphone needs to start the communication towards the gateway,  so in the router must be opened the port 64100 and the Internet connection needs a public IP Address. Also it should be possible the operator opens the port 64100 if the public IP is not available.
But I heard operators usually dont help so much regarding port forwarding on their network.
The gateway provides also the DDNS service, so there is no need of a static address.
I would like to install the system but at the moment there is no fiber no cable in the place, so I want to buy a SIM Card.