Twilio charges a relatively low monthly fee per SIM, starting at $2/mo. See: https://www.twilio.com/wireless/pricing

That is relatively low, but definitely not low enough in my case. The price point I'm thinking of is ~$25/year; giving 96% of my revenue to Twilio would not work. The device in question would be on wifi, so I really only need the mobile data to communicate in the rare event of wifi failure. And even then I don't need much data.

Have you checked https://hologram.io/iot-sim-card/ ?

With global roaming? Maybe something like knowroaming, $8 for a day's unlimited data in 100+ countries. Not much good if you want to use 100kbytes a day, but for your use case it might work

https://www.knowroaming.com/order/data-packages/160-global-u...

Roaming isn't necessary. And I want to use something like an average of 100kB per year, with many devices having a long period of no transmission (and being off network if that helps).

Can you not use a PAYG sim? In the UK Three will give you a sim you have to use once every 6 months (a ping would do), charge £1 per megabyte.

If you happen to use Project Fi for your phone service, you can get up to 10 data sims for no monthly cost, and data just comes out of your main bucket. Obviously not a solution for non-personal projects.

unfortunately, no carrier will do this. I suspect that this is because of administrative fees associated with simply allocating a phone number and the network overhead from having a device associate with cell towers when idle.

You don't need a phone number if you don't need to be called on one, and you don't even need to be actively tracked in the home location database if you don't need to be reached at low latency at all. Think low latency=incoming call, no loner low latency=hourly email check. The remaining overhead is just renewing the sim once it degrades and keeping database space to allow it to associate if it turns the gsm module on.

I'm pretty sure all registered SIMs (even data-only) on a carriers network get an allocated phone number -- at least that's what I'd always thought.

Is it possible for a SIM card to not have a phone number? My iPad has a SIM card for cellular data and it has a phone number even though it doesn't have a phone app.

That would be ideal in my case. It's data only and I'd be happy to leave the GSM module off 99% of the time if that made things cheaper.

See https://www.quora.com/Do-all-SIM-cards-come-with-a-phone-num... for someone who seems to know a little more than I do telling us that the number is not necessary for the card to work.

Knowroaming.com Essentially worldwide coverage, cheap rates, unlimited plans as well, no monthly fee, and only kicks in when you need it :)

Are you sure about that? As far as I can tell, the plans are all for limited periods.

Yup. You can purchase unlimited packages in a large selection of countries. Been using the service for years now. Amazing. And no need to swap Sims, or replace your home sim

There are a few: Freedompop, Truphone, Tello. But they all have minimum activity requirements so they wouldn't necessarily work as a backup.

Hologram.io is a $1.50 a month fee, but it is suited much more to IoT global roaming