Thanks for the suggestions. I am glad to see that there’s potential.
I just found out that, while I still retain DSL, I actually (to save some 5 EUR per month) asked the carrier to disable the phone number connected to the landline.
I will ask to get it reactivated and one it is up, I will try it.
My relatives all have landlines and, whenever I travel to visit them, I carry along a HP Jornada 720 that has a 56kbps modem. It is the perfect machine for a quick remote console visit in case of need. But it would be perfect if it would work from my Librem 5, giving me out-of-band access anywhere in the world.
edit: I plan to connect the modem to one of my main server’s serial port with console redirection set in BIOS, or perhaps just set tty. The modem handles authentication fine, can even have multiple users. This should be enough for my needs.