Now try that with a bank/vendor app. Or any other communication app. Nowadays, many don't even put the message body into the notification anymore, so you can't forward it via another channel (e.g. via SMS).

b) These systems are a general pain when you need to deal with human support if your number on file doesn't match the number you call them from. I have a few different numbers that I use variously and run into this every so often - I get a barrage of extra verification questions if I call from a non-matching number, callbacks seem to happen randomly between my number on file vs. my number listed in a ticket, etc. My wife teases me about constantly breaking systems due to hitting untested edge cases.

(Tangentially, various systems will require you to input a phone number with no information stating that it must be a number capable of receiving texts, and at some future date will try to send your landline authentication codes via SMS.)