To make an SOS call,  press and hold (Long press) the SOS button for 3 seconds, the watch will dial the 1st SOS number, if no one answers it will then call the next SOS Contact listed, it will move onto the 3rd number listed if the call remains unanswered.  It will repeat this process twice for all numbers, if the call remains unanswered.

Mobile operators settings can vary and change so voice mail should be turned off to ensure your voicemail does not count as a call being answered.