internal int RemoveCalViewModel(CallViewModel callViewModel)
 {
     lock (CallsViewModelList)
     {
         CallsViewModelList.Remove(callViewModel);
         OnPropertyChanged("CallsViewModelList");
         return(CallsViewModelList.Count);
     }
 }
        internal void AddCalViewModel(CallViewModel callViewModel)
        {
            if (FindCallViewModel(callViewModel))
            {
                return;
            }

            lock (CallsViewModelList)
            {
                CallsViewModelList.Add(callViewModel);
                OnPropertyChanged("CallsViewModelList");
            }
        }