My grandmother suddenly received a short message from a strange number, saying that I am Yuanyuan (my nickname). Is adding me to WeChat a fraudulent phone call?

This must be a scam. You have never sent such a message yourself. If it is really from your family, it is impossible for your grandmother to add your name. Now those liars are hateful. Your family's nickname, even some trivial things at home can be found out by all means, so the awareness of prevention must be improved; A person should never intend to hurt others, nor should he forget to watch out for the harm that others may do to him.