Getting error message, trying to send session-less SMS on CCAIP.

We keep receiving this error message:

“message”: “phone number "from_phone" cannot be registered for session-based SMS”
as a response on Postman.

Not sure what the error means and why it is related as we are trying to do a session-less SMS. The from_phones, phone number is in the correct format “+1XXXXXXXXXX”

https://cloud.google.com/contact-center/ccai-platform/docs/SMS#UUID-7277df40-2eed-42c3-4ec6-3bf1dda645a1