> Also, hard to offer a high availability option when the phone network only (well, in most cases) accepts one device per phone number.

1. I guess it depends on your providers/region. From all three German mobile network providers (Telekom, Vodafone, o2) you can get up to three SIM-Cards for the same number.

2. The VoIP provider Sipgate (sorry again German) gives you as much SIM-Cards and eSIMs as you like (In exchange for money of course). You can route mobile as well as land line numbers to a VoIP-Phone, -Client or mobile phones. They can all ring in parallel.

3. Many years ago, I saw a presentation on a CCC event. (Sadly I can't find a video of it just now.) It was from a guy who documented how he became a mobile provider. He wasn't just reselling, because his numbers terminated in his own Asterisk server! So maybe, people looking for the best solution, should look into how to become a virtual mobile provider.

I suspect they’re still used for outbound scam calls/texts (and maybe inbound too), and probably gray-market voip-pstn interfaces in countries that make int’l voip interchange expensive.

Some cool stuff on aliexpress with 128 SIM card slots and 8 or 16 gsm radios where you can program your choice of imei.

As a Canadian with crappy cellular coverage, I’ve dreamt of having a couple French SIM cards that I could mail to France every so often so it looked while I wasn’t 100% roaming just to have a cheap unlimited data plan with cheaper int’l calling.

Meanwhile, eSIM has come into existence, and removes the annoyance of dealing with the physical SIM card.