Clara can request a backup phone number to add to the invite description for all meetings. You can update this feature in your preferences dashboard.
If a phone number is not provided, then Clara will request a backup phone number in all followups until the meeting. As the meeting date approaches, Clara may followup more frequently than your Follow-up Frequency preference to improve the chances for capturing the phone number. If participants tell Clara they will not provide a phone number then Clara ceases to request one.