Subscribing means that all of the messages should show up on your other devices that are signed in. There's a problem with subscribing.
The problem is only oneway, however.
All of the messages from my phone show up on my other device (iphone). I can respond from there. even though I am not "subscribed". But the messages I send from the other phone don't show up on my flip. It's not a critical problem for me. But it's annoying.