For drop-in, you don't need to use any of your contacts or even give Alexa permission to read your contact list.
Assuming that your Echo devices already has names (rather name Echo 1, Echo2, etc.) all you have to say is "Alexa, drop in on [name of device]." For me, I have half a dozen Echo devices and even an Echo Dot in the car. The name of the Echo Dot is "car". So if my gf is driving in the car and I'm at home, I can say "Drop in on car" and I can speak to her while she drives.