Who?

Sammy and Jack were on the phone last week.  (Brian and Jack were gone for the week.  More on that later.)
After about 10 minutes on the phone, this is what I heard:
S:  "Are you my brother?"
Quiet
S:  "Are you my BROTHER?"
Quiet
S:  "Oh good.  I didn't know which Jack I was talking to."
(He does not know any other Jacks!)