If you're a parent and you've purchased a USA sim card for your son or daughter and you can't get through to them, it may be because of a number of reasons.
For example, it might be the middle of the night in the USA when you try to call them and their phone is turned off (remember that that the USA is a different time zone to Australia). We recently had a worried father ring us complaining that his son's USA sim card was not working, but it turns out he was trying to call his son at 4am USA time so no doubt his son was fast asleep in bed with his phone turned off.
Please note that voicemail needs to be set up by your son or daughter on their phone. If you call them when their phone is off and they haven't set up voicemail, you'll hear a recorded message about being unable to reach them. Please refer to the activation instructions about how to set up voicemail.