How do I build a network starting from the telephone connection?

To each their own, I say, because anyone who notices DECT stations should build a metal grid around their house so that the cell towers, amateur radio operators, etc. don’t also radiate into the house.

Joking aside, since it wasn’t mentioned above, it is also important to make sure the patch panel supports CAT6e.

When wiring CAT6a, it is also important that the individual pair shielding for each pair is carried to the terminal. Because if everything is CAT6a or higher and the connection is done correctly, later on, when the prices are reasonable, the 1GB switch can simply be replaced by a 10GB switch and the whole network will run at 10GB.

Regarding the DECT phones, perhaps they should remain switched on so that one can be reached? At least mine stay on for that reason.

Of course, anyone who says from 10 p.m. to 8 a.m. that no one is allowed to call because they don’t want to be reachable and who switches off their phones during that time and also isolates their bedroom for beauty sleep should be granted that, but they should then not forget to turn on/off each individual phone/mobile in the whole house every day, otherwise one might suddenly be unreachable for a whole day. (I would find it too annoying to run to the basement/ground floor/upper floor as the last thing every evening and the first thing every morning.)

Since I just see that I myself once spelled it incorrectly:
there is CAT5(e) to buy, with CAT6 the addition is (a)

Hello,

I work in IT consulting myself and would recommend the following additional points to most laypeople:

-When laying cables, always try to use the highest possible category with the best shielding (price not significantly more expensive but open for new speeds) – meaning install Cat 7; price for 100 meters Cat 7 vs Cat5e, approx. €60 vs €25
-Plan a Cat 6 patch panel for all cables in the house or network with switches (8-port from Netgear approx. €35), e.g. 1 x switch for the upper floor, 1 x for the basement, 1 x separately for the office/technical room (this has the advantage that later on certain lines can maybe be "prioritized" or throttled). Depending on where the telephone connection is located, the setup may change accordingly
-Use FritzBox + Fritz Repeater for WLAN "extension"; FritzBox (7390) can be configured to turn WLAN on or off, if desired a detailed weekly schedule can be entered (same applies for DECT or telephone)
-FritzBox has USB ports for printers and USB drives that all network users can access; the FritzBox media center even streams music and movies from the drive
-More network storage can be provided by a NAS, but please note that a NAS does not replace a backup (in case of doubt, buy 2 and place each in a different location)

To more advanced users with time and interest I would suggest:

-Cat 7 everywhere in the house, depending on what the electrician would use (cable price is very cheap, installation is expensive)
-Anyone who has the money should lay a cable from every room where it seems necessary to the technical room; please don’t forget that the fridge, TV, and talking toilet paper holder all want to go online (realistically: at every spot where someone might want to work with a laptop); possibly reduce the amount of cables (meters to be installed) through switches
-Why cables and not WLAN: I am in IT security and would only use WLAN when absolutely necessary and only on unimportant devices (might be considered a bit paranoid, but then again I hope I don’t live near you)
-WLAN repeaters, if necessary, build from old routers with DD-WRT; suitable routers can be bought used for €10-20, compared to Fritz repeaters approx. €70 new and can do 100 times less
-Build your own server for NAS and (smart) home services: makes everything much cheaper but requires expert knowledge

Regards