Some types of calls do not leave a missed call entry (ie. when call is answered elsewhere, or when this is a queue call).
If you provision this with the default 3CX templates, and make a call from Extension 100 lets say to Extension 200, and hang up while it is still ringing, then it will generate a missed call on 200. You can then open your webclient for Ext 200 and see if there are missed calls there (which should also appear on the phone if it rang).
You can factory reset and reconfigure if you want to be sure you have the default settings, and it should show the same missed calls provided the phone has rang.