public ConnectionViewModel
{
    private readonly CollectionView _phonebookEntries;
    private string _phonebookeEntry;

    public CollectionView PhonebookEntries
    {
        get { return _phonebookEntries; }
    }

    public string PhonebookEntry
    {
        get { return _phonebookEntry; }
        set
        {
            if (_phonebookEntry == value) return;
            _phonebookEntry = value;
            OnPropertyChanged("PhonebookEntry");
        }
    }
}

The _phonebookEntries collection is being initialised in the constructor from a business object. The ComboBox XAML looks something like this:

<ComboBox ItemsSource="{Binding Path=PhonebookEntries}" 
    DisplayMemberPath="Name" 
    SelectedValuePath="Name" 
    SelectedValue="{Binding Path=PhonebookEntry}" />

WPF ComboBox Binding