I am wondering if

If I have an SMS sender in my app, will it work during testing? At what point will it work? Will it work in the Adalo previewer?

It will work to send right from the start but obviously an SMS can only be received by a mobile phone (nothing to do with whether your app is on it).

Hello, as @speakupboy has mentioned, receiving an SMS depends on the phone number, so sending SMS would work for sure once you complete its configuration in the app and start the testing.

Thank you!