Why don't you start by enumerating the ChatMessageCollection 'MissedMessages and observe what you are finding:
private Skype skype = new Skype();
private var missedmsgs = skype.MissedMessages;
foreach (ChatMessage mmsg in missedmsgs)
Console.Write(mmsg.Sender.DisplayName + " " + mmsg.Body + Environment.NewLine);
Note: it's been a while since I used SKYPE4COMLib: wasn't Skype going to disable some facilities in it in 2013 ?