Sadly, Clef is shutting down in a couple months. If you haven't heard of it, it was a clever way to use your email and a mobile app to sign in to websites. I had integrated Clef logins to indieauth.com as one way to authenticate your email address. Since they are shutting down in June, I am proactively removing it from the website right now.
Many people keep asking me why I use Tropo instead of Twilio. It seems that Twilio does a great job of getting the word out about their service, so many people end up using them first. But there are a few reasons I'm using Tropo instead. Basically it comes down to features, scalability and support.