Don't get me wrong, I wasn't trying to debate buy vs. build. Although if I was going to buy there are much more robust services also built on-top of Twilio.
Just the op asked "what makes this different" and I was pointing out that the barrier of entry is very low.
With that said, if you already know the Twilio API and you already know AWS Lambda + API Gateway + DynamoDB it might be worth it to build because:
1. It would be extensible for your needs.
2. It would be a heck of a lot of fun if you enjoy that sort of thing.
Edit: I can speak to point #2 personally. I have done it. It is a lot of fun... especially for a developer who usually lives in a web browser... to be able to call a phone number and hear the result of your code. It is very satisfying. Much like building an Alexa skill is or finally getting that LED display to light up on your breadboard and print something.