Incoming Calls Routed To Someone Else

Today I received a call from a lady who kept asking for a guy named Shane. I am not Shane so I assumed she had the wrong number. She would dial a different number and somehow it would come to me. After the 3rd call I asked her for her information so I could try to call the number she was trying to call. I called the number and it went to one of my clients phone numbers who has nothing to do with the person she was calling about. How does this happen?