I would agree Siri is far from perfect but unfortunately you're using Siri like a simple voice dial feature when Siri is a little more intelligent than that. Like NotAdvisable said, you should just create one contact for your father with all his contact numbers. I have my brother set up with four contact numbers. If I want to call him, I can tell Siri to call my brother, and Siri will ask which of the four numbers to use. It will ask whether to call his home, or work, or Google Voice, or iPhone number. If I want to be more specific, I can tell Siri to call my brother at his <specified> number.