Voicemail should not be activated on the SIM card since this prevents the tracker from picking up the call when you call it for tracking.
Please contact the provider of the SIM card and have them deactivate the voicemail or call forwarding feature of the SIM. It should take effect within 24 hours.
When that is done, just turn off the tracker for 1-2 minutes then turn it back on and put it in an open area for 2 minutes before try calling it again.
It should ring once or twice then disconnect or you will hear a prompt saying that the voicemail has not been configured then disconnect.