SMS service is not active by default, activation is done at Marketing / SMS.
SMS is sent from an automatically generated phone number. You can only enter your own number in the text of the SMS message.
Prices for sending an SMS message can be found in the price list here.
The SMS payment is made by subtracting the credit from the wallet at the moment the SMS is sent. Since SMS sending can be set automatically after a change in order status, always keep enough money to avoid a sudden depletion of your credit and to block your eshop.