The String Phone Without a String

Nobita has thought of a funny joke and wants to tell it to Shizuka over the phone, but cannot use the house phone as Mom and Dad are using it to chat with their friends. Hence, Doraemon gives Nobita a String Phone without a String and gets him to to give Shizuka one end of it. But when he calls up Shizuka after a massive detour to avoid telling Gian and Suneo the joke and keeping her waiting, Nobita forgets what the joke was.