I don't quite understand your question. But as far as I can tell, the sms is sent on behalf of the current logged-in user. You can specify the user's direct number as the "from" parameter and it should work.
I have a list of company numbers, they don't contain extension but when I send messages from RingCentral phone application then I can select one of them to make the output. How to authorize and send sms from this numbers.
Is it possible to use any of the company numbers as the caller id for any of the extensions? Essentially having a many-to-many relationship between extensions and company numbers? I have a requirement to choose an extensions from number on an SMS text based on the recipient (basically making it look like the message is coming from a local area code).