The critical item is not the phone itself it is the telephone number assigned to the device that is essential. SMS messages are addressed to and from an active telephone number associated with a device capable of sending and receiving SMS text messages. If your big heavy theater phone is SMS capable and text message is activated on the account associated with it you can use that number. The device does not have to be physically colocated to your iMac or iPad to use WhatsApp although it would have to be present for the initial set up.

If we knew what it was we were doing, it wouldn't be called research, would it?

— Albert Einstein