GrapheneOS
The thing is, I've been using this carrier for a long time, and when I was using stock setup (admittedly this was on my pixel 3 not my current pixel 8) I didn't have any issues receiving any SMS including from FB and WA with this carrier. Also I have 2FA setup when logging into my carrier account and I'm receiving an SMS from them with my login code.
If I were to revert my P8 to stock google, are you totally sure that the issue would still exist?
I'm happy to try this because I'd like to understand the source of the problem, but don't really want to unless its totally necessary to find out.
In the carriers reply to me they write:
I have had a look at your account and postcode, I can see no issues at all. When you have issues receiving authentication texts from third party companies, you need to get in contact with the company directly.
They will then contact their SMS aggregator, then to the operator, and then to carrier that operator uses to get things checked out. We have no access to check this on our end, this needs to be investigated from the company you are having issues receiving the texts from.
If you are still having the same issues when all steps have been performed, please get back in touch and we will be more than happy to investigate this further.
I guess because Whatsapp and Facebook are both the same company, it's probably more likely that the issue exists with those services.