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.