Fixing Twilio ERROR 21610 When Sending a Test Message

I got ERROR: 21610 when sending a test message, so I'm writing down how I fixed it.

If you're on a trial account, go to the Verified Caller IDs tab and register the allowed number, and you can send messages normally.

I thought about emailing the company to fix this, but I solved it after reading a post someone had written on Stack Overflow!