Definitely, the deal is also officially with Denner. It is also visible after introducing a correct serial number

There are many on Android.

My preferences are:
Paid: Acrobits/Groundwire
Free: ACR Phone

There is no such restriction. As long as you pay the 0.99 euro/month the number is yours.

Of course, it can be used over 4G/5G. This is how I’m using it most of the time.

And the main page is just in french (it is a french company), but the account/management page is also in German/English

Yes, you get a Swiss number as you can see in the screenshot which can be used with any phone app which supports SIP protocol.

The main point of the “deal” is the Swiss phone number (the next cheapest is […]

1) No, the 3.99 Euro/month will give you an additional 1h of free calls to Swiss mobile numbers i.e. 0.05 Euro/min. The free 60 min per call is only to Swiss landline numbers. The 0.99 Euro/month gives you just […]

Sorry for answering in English.

I have never called more than 99 phone numbers during a month so can’t say 100% but AFAIU you get the special tariffs for the first 99 phone numbers you call during a […]

I bought a bike from them 6 months ago. You pay the CH tax as part of the order and that it is. You don’t pay anything additional at delivery. Similar to amazon.de.