Before implementing Infobip Rest API, Get this below values from Infobip and configure it in Site Property.

Steps: Factory -> Application -> Search Infobip SMS Connector -> Site Properties

And we can send SMS Message for "TO" number, TO number should be with country code (If you are using Infobip trail account you need to verify your numbers to send Message)

And then here you can add a new person and verify with OTP.

FYI, Also use your country code when you are sending SMS message and adding new person.

Phone Number Format - (Country code) Number

Added image for your reference (Ex)