SMS API restriction: existing phone number not usable with JWT extension (“Phone number doesn't belong to extension”)

Hi RingCentral Team,

I am implementing SMS functionality using RingCentral REST API with JWT authentication (server-to-server app).

Environment:

• JWT authentication flow
• Existing number available under Admin → Phone System → Phone Numbers
• Number assigned to a Softphone extension

Issue:

When sending SMS via API, I receive:

“Phone number doesn't belong to extension”

This happens even though the phone number exists in the account and is active.

Questions:

1. Is SMS sending restricted to phone numbers assigned only to the authenticated JWT extension?
2. Can existing numbers be reused across multiple extensions for SMS sending?
3. What is the correct way to bind or select an existing phone number as a valid SMS sender for JWT-based API usage?
4. Is there any configuration required to enable this use case without purchasing a new number?

Goal:

Use an existing company phone number as the SMS sender for OTP via API without creating a new number or extension.

Thanks in advance for the clarification.