That's frightening. I often see SMS delayed by an hour or more. When it's not a long delay, 5 minutes still happens often. I wouldn't want the Fire Services to be delayed so long

IMs have much more reliable delivery time - provided you're connected. But on the other hand, signal is routinely lost every time I go inside a shop or café. Neither IM or SMS provides reliable and timely delivery, unfortunately.

I agree that different tones would be very useful though.

You do get a different tone for IMs than for SMSs. While nice, I find that kind of annoying at the moment, because I choose between IM and SMS according to the person's connectivity, and use both channels for the same kind of conversations, so it's a bit mismatched that it uses a much subtler tone for IMs.

They've judged wrongly that IM == more casual. It depends how it's being used. For business conversations I use IM exclusively because with SMS the delay is too variable, and it might cost the other person.

Anyway, possible solution to your question: Use IM with all your friends, and SMS only from the fire dept + mother-in-law. Then you'll have a different tone for them. Or do it the other way around.