It's one device to one device (as stated elsewhere in this thread. Apparently, only Android+Chrome combo is supported for multi-device).

Put differently, I can't register the same number to an iPad and iPhone simultaneously at this time and hope to communicate with someone. It apparently gets things all confused.

I was doing some more testing and it also appears that I'm unable to call someone with a greyed out name but with available phone icon. Once again, no clear indication if this is a bug (not supposed to happen) or a feature (can call but not text).