"If you indicate a company number (not direct extension number) as your "from" number, you must be logged in as a company operator to be allowed to send SMS from this number. By default, main system administrator extension is defined as a company operator, but it can be changed in RingCentral Service Web UI . "

See the reference answer: https://community.ringcentral.com/questions/1557/send-sms-error-msg-304.html?page=2&pageSize=10&sort=oldest

Thanks Anirban, we already had found this reference, and we are logged in as our main company operator extension that is defined in our IVR settings. We have this changed from the default ext 0, to ext 222 AND we are using Multilevel-IVR, so I am not sure if either of these are causing an issue. But we are logging in as 222 and are able to authenticate and obtain incoming fax and sms messages for our main number, we just cannot send.

Check your IVR general setting to see which extension was selected to receives all calls and messages by default for the main company number. Login your app with that extension then you will be able to send SMS from the main company number.