Now for the no credit notification, if both iMessage and Facetime are turned off, then this might have something to do with Premium TXTs, or sending MMS. Just a guess really here, but is it possible that some service/content provider is trying to send you some premium TXT message which is chargeable, and so it's triggering the network to send you the low credit notification? (more details re Premium TXT: https://www.vodafone.co.nz/mobile-services/premium-interactive-txt/)
Again guessing here, but this might have something to do with Amazon and Twitter 2FA. It can be that either is trying to send you the 2FA txt and it's going to be a chargeable one and not free, and so its triggering the no credit notification
Would try and top up and see what happens afterwards, and if you would receive any TXTs at all from either service. I couldn't find a list on Amazon's website regarding supported carriers, but if all fails and you're unable to access your account for either Twitter or Amazon, then would recommend opening a support ticket with Twitter/Amazon regarding alternative ways to access your account.