I came across this thread on the T-Mobile forums.....
https://support.t-mobile.com/thread/148985
The problem there is the same as here. It is the sort of thing I had in mind, and what I had hopes of the customer service rep drilling down on.
Yes, it's T-Mobile, but it's the concepts I'm pointing at: Blocks on accounts, short codes.
If the problem was with one particular company, I'd say the company. If the problem is general, and the phone receives texts from regular phone contacts, I still think the problem is on the carrier side; maybe a block that the user set or requested, but on the carrier side as opposed to a setting on the phone.
I would call customer service again. I know it seems like a waste of time, but this is you best path still. Ask if there are any blocks enabled for your line. Mention the companies that you have requested send SMS to you.