In my case I have a data only plan with US Mobile (their “GSM” sim card), that works fine.
Then just for giggles I did a one time addition of 75 minutes of telephone time (for USD 2). I can make and receive calls just fine, but only when the modem is in the 3G mode, 4G does not work.
As far as “flying under the radar” goes in this case: US Mobile is fully aware that I’m using a Librem 5, I called them (on a separate matter) and discussed it with them, they told me that is fine, they didn’t care.
In case any of the above adds anything useful to the discussion…