If you live in Germany and have a Fritz!Box router, you can just buy a second old-ish Fritz!Box and a simple landline phone (from the likes of eBay, Kleinanzeigen...) and hook them up via WiFi.

Voilà, telephone service as it used to be. No proprietary payphone with questionable ToS and privacy policies needed.

Why does one need the second (old-ish) Fritz!Box? Doesn't the first one already have DECT?

Also, does this not require a landline number?