It is assumed here that your ListBox is called
ListBoxPhoneNumbers
your TextBox is called
TextBoxPhoneNumber
and your ComboBox is called
ComboBoxCountryCodes
containing values like this
"+31",
"+32",
"+33"
then you could insert the complete number like this:
var completePhoneNumber = $"{ComboBoxCountryCodes.SelectedItem} {TextBoxPhoneNumber.Text}";
ListBoxPhoneNumbers.Items.Add(completePhoneNumber);
See also
$ - string interpolation - C# reference | Microsoft Docs[
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