If your tracker rings for once or twice and immediately hangs up, the cause is most likely that you have not set the SOS number.
This video explains how to set your SOS number.
When you set the SOS number on your tracker, you're authorizing your cell phone as a number that is allowed to listen (otherwise, anyone with the tracker's phone number could listen in).
The easiest way to set the SOS number is to use the Logistimatics mobile app. In the app go to Commands > + > Set SOS Number. Then enter your cell phone number (not the tracker's phone number!)
Once this is done, you will never need to set it again and you can listen to live audio by simply calling your tracker's phone number.