That is mostly SMS fault. It doesn't guarantee reliable delivery either in theory or practice, and the carrier will happily drop messages if a resource limit is exceeded. Essentially all of the IP-based chat protocols are reliable, whether that is WhatsApp or iMessage.
It absolutely guarantees reliability on the protocol level. If mobile network operators are load-shedding, that's on them.
I've never experienced this in decades of using GSM, and it's only recently become a thing (mostly for 2FA SMS since I stopped using it for P2P communication).
> Message delivery is "best effort", so there are no guarantees that a message will actually be delivered to its recipient, but delay or complete loss of a message is uncommon, typically affecting less than 5 percent of messages.
