I'm following this tutorial here: https://docs.mongodb.com/stitch/tutorials/todo-twilio-web/
In Step E. Add your Twilio Phone Number as a Stitch value, it should be specified that the number should added as a string (in quotes), otherwise, it gets converted to an integer which Twilio will reject.
Also, I thought it would be cool to mention how you can test the function using the "Run Function as" feature, however, that fails for me with the "> error:
The number +14355696753 is unverified." I'm not sure where stitch is picking up that phone number, as my ourNumber variable has a different value.
Finally, I had trouble running the code as provided when I tried to submit my phone number for verification (the users variable referenced wasn't defined). I created the following pull request which fixes the problem: https://github.com/mongodb/stitch-examples/pull/41