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!
Avoiding roaming fees: forwarding incoming number via another provider to a foreign phone number or SIP-number
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