Dave got an Amazon Echo Show for his mother in law and he wants to know how to call her with his. How can he make calls from one Show to another? Leo says she has to allow it. He also has to be sure both of them are on each other's Show contact list. Dave will need to know the name of her Show. Amazon has a good setup tutorial on how to do Drop Ins here.
She also needs an account. If he bought it for her, it's probably been associated with his account, not hers. So he'll need to de-register it and then register it under her account.