Login to your status page at instatus.com/login then choose your status page.
Go to Settings → Notifications → Methods
Enable subscribe by SMS
Click Save
Go to Settings → Notifications → SMS Setup
Select either Twilio, Esendex, Vonage, MessageBird or SNS as your service
Click Setup Twilio, Esendex, Vonage, MessageBird or SNS
Send a test SMS to ensure that you successfully integrated your SMS service