A. First of all, either player on a doubles team can make any line call on their side of the court. It is a common courtesy to assist your partner with the service line call while she is returning though.
Set up at the service line when your partner is waiting to receive serve, and look to call this line as the ball lands. When you notice that your opponent’s serve is in, begin moving forward a little. If you recognize that it is a slow serve, or right to your partner’s big forehand, then close aggressively. On the other hand, if it is a particularly effective serve, then be cautious because you might need to defend yourself immediately. In other words, be alert. You should be able to both help with the line calls AND be aggressive around the net.