right, that’s why I assumed it’s a serial connection. But what device is meant to connect to it?

It’s probably not something I want to reverse engineer blindly. I was looking to find out if someone knows exactly what device is intended to connect to it and what info it gives. After getting the vague hint that a similar port on my boiler is apparently possibly meant for sending SMSs, I want to know whether this is worth my effort. If there is a cheap device I can simply buy, that might be interesting.