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
Elite includes 10 and Ultra 100 SMS per month from the plan. When that quota is used up you can buy extra SMS on the Subscription page; purchased balance does not expire at month end, stays until used, and is consumed after plan quota.
Plan-included SMS is 10/month on Elite and 100/month on Ultra (0 on Free and Premium). Extra SMS can be bought only on plans that include SMS. Pricing tiers: 100-249 SMS at USD 0.05/SMS, 250-999 at 0.04, 1000+ at 0.03.