When you begin to type in the person's name into the dialer, if they have more than one number then you should see the total number of phone numbers with an arrow pointing down to the right of the person's name as it appears in the dialer. You can hit that arrow & it will show all that contact's numbers.

 

In the contact hit 'Edit' (the pencil in the top right) then hit the 'link' (the infinity sign) and select the other contacts for that person. It declutters the list