iPhone not receiving OTPs

thank you for your suggestions but, sadly, none of that applies. I have service, I have a good battery (the phone is 2 years old), i have everything else working perfectly, including everyday messages. I just can't get verification codes when i need them! My Apple ID and message settings are perfect (my phone number is now fully verified). Just to pick one site as an example, I can access my Sainsbury grocery account perfectly well on my Apple Macbook - and curiously, it doesn't want to send me any codes - just not on my iphone Sainsbury App, which suddenly does!

perhaps i am being confusing; i don't have a problem (now) with my Apple ID. I received the 2factor authentication code to sign back in and all is well, but only after i connected to a mobile network rather than via WiFi calling. I also noted iMessage was now verified and i received a batch of old messages at that time. A few days after I returned onto WiFi calling, although iMessage was properly authenticated, I have once again stopped receiving authentication codes from financial institutions and some others, such as Sainsbury.

On Apple devices, once you sign up to 2 factor auth, you can't turn it off anyway and I can't change anything, such as to my home phone number, on my other accounts without a code.

No, this is the UK and there is no mobile signal where we have lived, for the last 7 years (Exmoor National Park), that's why ALL our calls and messages go (free!) through our broadband, using WiFi calling. We've had NO problems with any of this until recently.

ok. It doesn't explain why I've had no issues using my iphone over WiFi calling for any of this for the last few years but it seems most likely this is something that's changed during an Apple update because nothing else on my phone or my network provider has changed. I do know that I am not the only one with this issue and the widespread use of 2 step authentication is making life very stressful.

Thanks for trying.