Not receiving texts for two-step verification from various sites

I have had an Iphone X for a couple of years. I have a number of sites (brokerages, banks, ISP, ADT) that use the two-step verification where a text with a (usually) six digit code is sent to my phone. after sign-in. However, recently and for no observable reason, my phone has stopped receiving such texts. I have to tap the alternative (send e-mail) to receive the code, which works fine. One site sent a voice message with the code as an alternative, so it's not that their number was somehow blocked by my phone. Also, I receive all normal texts. It's just the two-step ones I'm not receiving. And as far as I know it's all of them.

I've checked my settings and don't see anything out of the ordinary.

Anyone know what's happening?

Thanks