In order to configure the plugin you need only to insert your fax and/or sms number. The plugin has an on-demand cost: you will be charged 0.16€ for each sms or fax sent.
☞ Docs: Plugins
Sometimes the fax does not arrive and generally it depends from the fax line. If the number corresponds to a voice + fax line and if the number is used as a voice line at the moment of transmission, the operation will not be successful. After 3 attempts the system will not send the fax again.