Do you know how to delegate number an extension to have access to main number to send SMS from?
I am afraid that this is not possible. Not even when an extension has the super admin role. The only way is to give that power user to login with the company main number.
Maybe a workaround solution is to have a server app that use password flow to login with the main company number. Then the user who can send SMS on behalf of the company (using the company main number) can have access to that app and send SMS with that main number.