Originally Posted by LuckyR
This may come as a suprise but the returner's partner's job is NOT to call lines, it is to be ready for the potential poach. Wherever they choose to position themselves is their choice and they try their best to call lines from wherever that happens to be.
From 'The Code':
24. Service calls in doubles. In doubles the receiverís partner should call the service line
, and the receiver should call the sideline and the center service line. Nonetheless, either partner may call a ball that either clearly sees.