You can get notifications as push notifications and/or email free of charge. If you prefer to be notified by SMS you can purchase SMS to the farm. Note that only the owner of the farm can make purchases.
1. In the menu select "Manage Farm" and scroll down to premium features.
2. Click on "Charge"
3. Select the number of SMS you want to purchase and finalize the purchase.
4. Now you have SMS available for the farm.