ADO.NET :: Newbie - How To Define A Composite Primary Key In A Model

I have a table where a primary key is a composite key build from columns: phone_nr and phone_ext.

How do you put a KeyAttribute in a Model in this case?

When a primary key is one column in a Model I coded:

[Key]
public string category_name { get; set; }
public string user_id { get; set; }

in the above case the column category_name is the primary key, but what to do when you have a composite primary key?