I wrote my own SMS gateway API with authentication tokens, quotas, rate-limits. This is because I wanted to send SMS without relying on an external API, so I got a 2€/month SIM card and plugged a USB modem (Huawei E169) into my RPi to use with Gammu. I'm using Gotify to log sent and received SMS, and send an SMS whenever my home internet is down or the IP address changes for example. It's plugged into my systems monitoring for critical alerts, and while I offered API keys to my friends, none of them wanted any so I'm the sole user.