Yeah, I’ve also use that to further segregate things by level of importance: if a person on the other side can’t be arsed to leave a voicemail, it’s not important enough.
The SMS option just allows further segregation of important-non-urgent from important-urgent: for me an SMS might have something I should know (say, confirmation of a doctor’s appointment) but have plenty of time to deal with (say, it’s in 2 weeks) so it works well for automated messages.
In the old days of WFH I would further segregate it by “if it’s really really urgent come to my desk” which further filters for importance based on the effort others are willing to put on coming to me with it…
In my professionally life I’ve concluded a lot of unecessary stress comes from unimportant, important and urgent and important but not urgent coming in via the same channels and me having to treat everything as “possibly important and urgent” when most of it is no such thing, hence my filtering by-effort-required, which is not perfect but works way better than most people’s approach to it.