But is this a message from Rick or a message to Rick?
It’s entirely unclear what kind of spirits he’d be in at this point, but I’m sure he’s had some low moments over the years without his loved ones around.
In my mind, whoever made it probably had contact with Michonne and Judith, or at least witnessed them from afar (perhaps from a low-flying helicopter), and then sent the message to Rick with a goal of keeping his spirits up.
It will likely end up being explained as the latter, since the phone wasn’t exactly left in an obvious spot for Michonne to find it.