"Who wants to call my grandfather?"
Translation:¿Quién quiere llamar a mi abuelo?
Quién uses the conjugation of él, ella, usted, etc. so it can refer to pretty much anyone, except for groups of people. Then it would be "Quiénes", and would use the conjugation for ellos, ellas, and ustedes, so it can refer to pretty much all groups of people.
I think it's singular because presumably only one person will do the calling?