You can't 'call' Interface methods since they don't implement anything.
That's why it's called implementing an interface.
If a class Employee implements an Interface IPerson, then Employee will have to implement all the functions defined in IPerson, don't know it it makes sense though.
You should read a little about Interfaces:
http://msdn.microsoft.com/en-us/library/ms173156.aspx[
^]
Hope it helps