Appears to be a new problem, wondering if it's working for other CiviSMS users. It works well when a contact is check-marked in search results and the action invoked from that screen. However if click on the contact and then select action "Outbound SMS" the "To" field is not pre-populated and non-functional.
CiviCRM 5.40.2, Twilio extension 1.2.1, Drupal 7.82