Here are the verbs in the correct form:

At 8 o'clock last night, Marion **was watching** her favourite DVD while her sister Georgia, 1) **was reading** a book. Suddenly, the phone 2) **rang**. Marion 3) **picked** up the phone and 4) **said** "Hello" but there 5) **was** no answer. Then the phone 6) **rang** again. This time there was a strange voice on the line. "I 7) **just arrived**." The voice 8) **said** "I 9) **will not forget** you. The girls 10) **got** so scared they 11) **started** screaming. Then the phone 12) **rang** again. "It's your brother Peter," the voice said. "Don't 13) **only tease** you! Let me in!" The girls 14) **started** laughing and they 15) **opened** the door for him.