Making this thread was a bad idea.
Really bad idea.
But that's forgeted up that a group of richardwads are spam calling your grandparents.
It's a skype call group probably. Considering you said that there were more than 1 voice on the phone.
Ask your parents to check the caller ID. Then you can contact skype support and find out who it is.
The skype number is what server it's coming from I believe.
Or
Call your telephone services provider and ask for assistance blocking the number, and/or your local police department. The police can trace harassing numbers if it is justified, including blocked numbers, etc.