We have an alert channel in Slack, but it's mostly ignored. Our primary alerts come via SMS/VictorOps.
At one of my old jobs, we had SMS via two physical/hardware devices in our data center. One had a Telstra SIM card and the other had an Optus SIM card. (They were plugged into the same machine, but we had plans to put a second one in another data center before I left).
If you really care about alerts, you should have physical hardware doing your SMS messages via two different point-of-presences.
