Sending a bulk SMS (to all your customers or many at once) costs 10 cents per SMS sent.
Go to the Marketing functions (to the right of Customers) and select "Create Campaign". First, you can enter an internal name here and must select the type "Bulk SMS".
You then have the option to edit the campaign and prepare the text. To do this, go to the campaign and select "Design". Test your SMS by selecting the corresponding link. You will then receive an SMS at your stored mobile number.
Once you have prepared everything, you can click "Start Sending" in the "General" tab. Sending usually takes 1 minute per 50 SMS.