Ambi Sitham told the court that she sat next to Rebekah Brooks at the meal in 2003 and that Brooks had suggested they keep in touch.
She told the jury she went to give her telephone number to Brooks.
"Piers [Morgan] turned to me and said 'Careful - she'll tap your phone'," she told the court.
Cross-examined by Clare Sibson, representing Brooks, Ms Sitham told the court she was "definitely a little bit concerned" by Mr Morgan's comment.
"I wasn't sure how serious the threat was," she added.
Brooks denies all the charges against her and the trial continues.
More top news
The winner of X-Factor Ben Haenow has revealed how keen he is to have the Christmas number one.