I recently retired from the local volunteer fire and rescue service. Several years ago the 911 dispatch service wanted to drop paging for notifications and move to SMS. I wrote a nice little technical critique of that plan. In addition to the basic issues regarding coverage (many members have no cell service in their yards, never mind in the fields, yet pagers basically just work), I learned from my technical contacts at the telco that there were a number of service guarantee problems. In addition to the lost and delayed message problem you discovered, things only get worse when crossing providers. As he put it, it's not so much that it works so good most of the time, it's that it works at all.
Dispatch did go with SMS notifications, but as an add-on to pagers and "robocalls" to registered phone numbers. We tracked notification channels for several months and found that with every callout, at least one member would report getting the SMS at least 20 minutes later than the page or phone call. Note that most members can get to the hall in 20 minutes or less. A couple of times over the years, we got a flurry of SMS notifications after we were on scene.
Friends don't let friends use SMS for urgent or critical communications.
For anyone not sure what this is all about, CBC has a pretty good podcast covering the descent into medieval nonsense.
I had relatives living just a few miles away in one of the hotspots.