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