TMobile used to offer (maybe they still do?) a data-only plan that was/is meant for remote sensing applications (alarms, etc). It wasn't something that was listed on their site (ok - it was there, but you had to really, really dig for it to find out about it).

IIRC, it was also free, but limited to 200 MB a month.

You basically got a SIM card you could stuff into your embedded sensor app and away you go.

That might be a large enough data cap for plain-text messaging, but not much else.

That may be impossible under current cell designs. WiFi hotspots even have assigned phone numbers.

You'll always have a phone number, but on an SMS- or Data-only plan it will never ring (MVNOs in my country offer these types of plans at a discount because reserving a voice phone number here is subject to ID verification and higher fees)

But a hotspot doesn't ring, so you can ignore it.

My hotspot gets spam text messages but I'm only aware of them the once or twice a year I log into its control panel.

Get a Wifi box, that itself contains a SIM card. Then you connect to it from your phone using Wifi. You just need to be careful because your phone will now take the liberty to update apps and use your data more liberally.