pic added to the OP

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 meant for sending SMSs, I want to know whether this is worth my effort.