If your SIM card has data roaming activated by the operator it will be active.
When home operator is not avaible, device will switch to another operator. If currently used roaming operator is included on roaming operator list, device will send data according to roaming settings in data acquisition. If currently roaming operator is not on the roaming operator list, device will use settings for unknown mode in data acquisition settings. If the user wants FMB125 to not connect and work with a particular operator it has to be written to Operator Blacklist.
Please visit Teltonika Wiki page for more detailed informations: