Twilio provides an API and platform that can send SMS texts, and text-to-voice messages, and costs very little to use.
Some 3rd party vendors already have hooks into Twilio — e.g. WrightCCS for two-factor authentication. If you want to roll your own, however, then creating an app in C# is very simple using the examples here.
Such an app can now be called in a variety of ways. I’ve personally implemented it in the Frameflow monitoring tool and VisualCron — an excellent Windows task scheduler. These were of particular importance for managing alerts when the application impacted is Microsoft Exchange (because obviously no emails alerting IT Support could be sent).