The device can, during normal operation, send notifications with information about the status of the output relay, and then the SMS will indeed be charged. However, in order for this to happen, this function must be enabled in the controller. By default, it is disabled.
When configuring the device's settings, SMS messages are indeed sent back informing about the status of the execution of the configuration command sent to the controller. But if the PrePaid card runs out of funds, the device will stop sending back these messages, but the commands will be executed normally (except for inquiries about the status of the device, numbers stored on specific positions, GSM network signal strength, etc.). - i.e. all those that require sending back an SMS with the specific information being asked of the controller). It will still be possible to configure the device, such as adding and deleting numbers in the authorized list, but the controller will not send back confirmation of the command execution.