How to pass a optional parameter into ActionResult

This works:

public ActionResult Edit(int id, CompPhone cmpPhn)
{
  var vM = new MyViewModel();
  if (cmpPhn != null) { vM.CmpPhnF = cmpPhn; }
  ...
}

If I make cmpPhn optional:

public ActionResult Edit(int id, CompPhone? cmpPhn)

I get "Error 1 The type 'MyProject.Models.CompPhone' must be a non-nullable value type in order to use it as parameter 'T' in the generic type or method 'System.Nullable'.

How can I make this input parameter to the method optional?

Here's the view model

public class MyViewModel : IValidatableObject
{
...
public CompPhone CmpPhnF { get; set; }    
...
}

Calling method

[HttpPost, ValidateAntiForgeryToken]
public ActionResult PhoneTest(MyViewModel vM)
{
  if (ModelState.IsValid)
  { var cmpPhn = vM.CmpPhnF;
  return RedirectToAction("Edit", new { id = vM.AcntId, cmpPhn });
  }
  ...
}