I have the fritz box but haven't used the phone facilities, I just plugged a very old dect phone base station into the wall socket via a microfilter. This thread reminds me that I should really have a poke around at what the router can and cannot do. I will report back if I find anything useful.
Zen have their own fibre backbone, and seem to manage traffic and contention really well. We see 76mbps, the theoretical max, on FTTC, the cabinet is very close, and can easily stream 4k to 1 device and HD to another.
One day we will all need faster connections, like when the local hospital sends its robot surgeon to you and they work it from their office. But for now it's fine.
I do have one of the Zen phone packages, UK landlines for about £7 a month, but rarely use it. Mobiles for most things. Zen don't charge the connection fee for in bundle calls, or at least not on the version I have.
The house is L shaped and a bit rambly, plus detached garage which is my workshop. Mobile signal here is patchy so we use wi fi calling. I bought (not from Zen, just online) a fritz 3000 mesh extender. Router at back of house downstairs, mesh repeater at front upstairs, rock solid coverage in all of house and garden.
Just to give an example of their service, one of the 2 times I had a problem, I was getting intermittent drop outs on WiFi but ethernet was fine, router a bit over 2 years old. I rang for advice, thinking I might buy myself a better router. They told me they had heard of that model router running hot and the chipsets giving faults, so they would send me a new one... came next day.
They aren't the cheapest, but its such a big part of our lives now especially if we do some work or study from home.