As for the calls to the SIM card used in the tracker, we can only assume that most likely the issue is with the SIM card.
It is better for you to contact your mobile operator directly to check the operation of the SIM card, traffic and limits, as well as mobile network coverage in the place where the tracker is located.
At the same time, issues in the work of the tracker itself cannot be ruled out.
If you are using our app with this tracker then write to us to email@example.com and send us your username (login), photo of your tracker, manual (if you have one), as well as screenshots of SMS commands and the device’s responses to them. We will check additionally.