Just made a quick google search: indeed, it seems that SALT is the only provider offering WIFI-calling abroad without roaming fees (see the box): Wlan-Telefonie im Ausland - Gesalzene Salt-Rechnung wegen Wifi-Calling in den Ferien - News - SRF. The article itself is about another problem, which, if one knows, isn’t really a problem. I guess this WIFI-calling (which as @zrher confirms is obviously also a 3G/4G-calling) seems to really be a great option, however, for me, only if it is available as a prepaid option!
Yes, WiFi calling works for incoming SMS as well.
“Wifi Calling using cellular data” is not advertised anywhere. It’s difficult to find information about it on the internet.
The first step would be to get a pre-paid SIM card and test WiFi calling while in Switzerland. Enable the feature and connect to WiFi. I think you can even be in airplane mode to be 100% sure that your calls and messages are indeed routed through the internet.
The second step is to prevent the SIM card to register with a foreign network, as I explained before. Once you have internet from your 2nd SIM card, the first will register to WiFi calling using cellular data.
This works for me on iPhone 11. I believe any newer iPhone should work. Not sure about Android.
Thanks again for the detailed explanation, much appreciated!
I’ve actually got a Salt prepaid card to test. Unfortunately, the Wifi calling option doesn’t show up in the settings menu (it does on my Yallo and Aldi mobile SIM, I’m using Android). Anyway, I’ll stop by a Salt shop today and ask why.
If it works, I’ll test it tomorrow when I’m abroad. There’s one thing I’m not sure about with Android: here in Switzerland I can force the phone to use the Salt network only. However, as soon as I take the SIM out of the phone and put it back in, it automatically searches for a network, and when I’m abroad, it’s obviously not possible to manually select the Salt network, as it doesn’t appear there. The only option then might be to select a network with which Salt doesn’t have a contract. To avoid this, I may have to insert the Salt SIM card before I leave Switzerland.
I will let you know about my experiment 
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Any update ? I am looking to understand on my side how To do as I am relocating abroad
While I do not know how to forward calls from an existing number, there are services like Skype that allow to create a local swiss number and use it as voip number
Get a local phone number & answer calls on Skype from anywhere | Skype
I recently discovered an App which was founded not so long ago (ex Adyen techies) which was launched precisely to tackle this issue:
https://www.firsty.app/ - you can also just download it from the App store.
I’ve downloaded it but haven’t completed the set-up yet as I’m also in the midst of changing mobile providers.
Anybody here experienced with Firsty?
If it’s successful, it will pull the rug out from under the cash cow for a lot of companies…
It’s not true unlimited:
Meaning for 2.50 euro a day you get 2GB of unthrottled data (and propably worse video quality as written in the text) which is not that much when swype offers 10GB a day for 3 swiss francs.
Firstly is probably not interesting if you want fast Internet. Firstly is more interesting if you don’t want to spend money on roaming and only need WhatsApp etc. Firstly’s free offer is sufficient for this, but it is too slow for everything else. I tested it once in Switzerland, but it’s already been a few months. The esim was from a company outside Europe (united arab emirates or similar), in Switzerland Salt was used as roaming partner.
It doesn’t seem that bad for random holiday roaming. Can it be considered safe? I always wondered if they can for example intercept encrypted data (gmail , banking app etc).
Note: That would be the same issue I’d have with a eSim only provider though
Could be interesting to try out.
Don’t think there is much of a risk. Those platforms are all by default HTTPS encrypted and if you like some additional security, you can use a VPN*. Personally, I always use a VPN outside CH or in public wifis, even though this of course requires you to trust the VPN provider itself.
*Only makes sense if you anyway have a VPN subscription already. If you however have to buy a VPN subscription for this purpose only, it defeats the purpose of “free”.
It’s nice if you get the free version (and if you have an eSim phone).
Otherwise you have plenty of options for cheaper esims. The only positive thing is that you don’t have to search for an offer every time you travel.