Hi seonho Cha,
Twilio gives an option for 30-day limited trial. So, you can use that initially for development and later on buy it.
If you have any query related to Twilio; please reach out to Twilio support.
Please use twilio user guide for setting up the configuration: https://marketplace.thingworx.com/-/media/Marketplace/Download%20Item/Related%20Content/T/Twilio-Extension-User-Guide.pd…
Here are some tips:
1. We need to use the twilio number for sending message. Our personel number will not work. If you want to use your personel then you will have to port it to twilio.
2. In trial version; we can send message only to verified numbers. So, you will have to verify your number on twilio before attempting to send a message.
I hope it helps.