I think you can do cat 3 cabling. Only designed for 10Mbps, but that's different from DSL tech. The phones lines were never designed to run any digital data signals, but here we are.
I've seen them use cat 5 too.
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I've used a couple of the older models and when i say used i mean reset, reconfigured and almost deconstructed physically.
A lot of establishments in Tenerife use fritzboxes for Wi-Fi I've found they have a tendency to ignore lease times, resulting in stacks of 254 devices locked into the router preventing any new devices connecting. Also config had no blanket removal of Mac addresses so it's either factory reset or device by device removal.
Not an issue for home connections but a real problem for businesses with a lot of transient custom, such as bars.
Hoping new versions are better