Keyless access systems

Doorbell button, relay module, door buzzer, IP cam – nothing more is needed for a basic setup.

Can someone recommend a simple door intercom system (with a/b interface for the FritzBox)?

If you can build everything yourself and have the time for it, you are usually cheaper. But is that an argument? After all, you have to factor in the time. It's the same when you install and configure KNX yourself. I did that too, but sometimes you have to know when to stop tinkering.

And as I said: the thing can do a lot more, for which you would need a server/Raspi again with an IP Cam. And then the time factor comes into play again.