I've posted the guide in other comments.
To get to Cassandra, you first must get eaten by Amy at the pool, talk to her right after she leaves with you inside her, she will give you the location of Amanda's shop. Escape from Amy's stomach, takes about 4-6 struggles throughout the day. Go to Amanda's shop and buy the phone from her. Access the phone by clicking the lower right of the screen, click on contacts, Cassandra will be one of two contacts.