When SMS?
When you choose SMS as the channel for client messages (or when WhatsApp is unavailable), messages like confirmation/reminder can be sent via SMS. You choose the channel in Settings → Messages & Notifications.
Quota
The Ultra plan includes 100 SMS per month; when the quota is exhausted no new SMS is sent and usage is reflected in analytics.
The SMS quota exists only on the Ultra plan; on lower plans client messages are sent via WhatsApp.