IMPORTANT: Nexmo calls are not free. You should pay on per-call basis according to their voice pricing. This shouldn't be a huge concern unless your website goes down twice a day.
- Create accounts at https://uptimerobot.com/ and https://nexmo.com/ (you'll get €2 welcome credit from Nexmo).
- Add your phone number to Nexmo, then get your API key and secret at settings.
- Go to UptimeRobot settings.
- Press "Add Alert Contact" and choose "Web-Hook".