"Following the same instinct that had prompted me to lie to Mike, I called Jessica on the pretense of wishing her luck at the dance."
This occurs in chapter 12 (Balancing) of Twilight, on page 219. I do not understand why Bella would want to call Jessica, if she is going to lie to her anyway. What does this mean here?
Before this happens, Edward explains to Bella that he worries about what might happen between them. This is why his family is cautious of the relationship, too: if things go badly, and Edward can't control himself around Bella, then they will all be in danger of being exposed. If people know that Bella and Edward are going to be alone together, and Bella disappears (because he kills her), then Edward will be the likely suspect and his whole family will have to flee.
Edward tells Bella this because he wants her to be careful around him, and he wants her to be sure that people know that they are going to be together, to keep as a backup plan and to keep him honest. But Bella goes in the other direction, lying to people about spending time with him alone so that no one will suspect him if anything happens to her. She lies to Mike to tell him that she is staying home to study and do laundry the weekend of the dance, and Edward will be out of town. Then she tells Jessica that her plans with Edward have been cancelled, too. She calls Jessica so that there will be more than one witness to say that Bella had no plans with Edward that weekend, in case something goes wrong. In a strange way, she's giving him a way to escape in case he murders her.
Bella actually rejected Mike in a kinder way to not hurt him and she is trying to lie to Jess to say she is not going out with Edward. This way no one will suspect them and if it travels back to her father it will be safe.
In this way I think Bella is handing Edward over a lot of trust. When Bella lied to Mike it was to cover up the fact that she had plans with Edward and to also reject him in a kind way. However in this sentence it illustrates that she is going to lie to Jessica for the same reason which is to cover up the fact that she went out with Edward. Also she does not want Jessica to go around gossiping saying she went out with Edward then it suddenly gets back to Charlie who she also lied to about her plans for the day Edward and herself have a date.