Before you activate automated SMS, you must go to the Billing section and fill out your invoice details.
Now you can click on "Buy Credit" and choose your preferred credit amount, select 20 Euro, 100 Euro or 300 Euro.
Enter your credit/debit card details and complete your payment.
Don't forget to download your receipt.
Also, you can see the cost per SMS to each destination so search for the countries of your most common guests that stay in your property.