I use my RUT850 with a German Telekom Multisim. When the RUT850 is powered on, it's blocking all incoming calls on my main SIM. I learned that there is a setting in other Teltonika devices where you can edit this behavior:
Services -> SMS Utilities -> Call Utilities -> Reject unrecognized incoming calls: Uncheck the checkbox -> Save settings.
This setting is not available in the most actual firmware for the RUT850. I also learned, that this problem exist for at least one year and that It was already reported to the RnD Department...
I would be very happy to find a solution, with the actual behavior the device is useless for me, as I´m not able to predict when someones trying to call me