Hi,
UK here; we had this. Very similar.
Firstly: NO message or email or popup about any device added to account and we checked no devices appear listed either. so that is different.
But; 3 or 4 messages with just "1" to Philippines followed by 40 spam chinese messages
Strange thing; the iphone wasn't even signed in to iMessage with the Apple ID... The iMessage was just set up as a phone number it seems...
So we can't see any unusual activity on the Apple ID account in question; but how were they able to send messages from this iPhone then if it wasn't set up for iMessage on an Apple ID!? (More to the point; since when could you use iMessage without an Apple ID anyway since this is how my partner had her phone set up)
Something strange is going on here. This isn't a compromised Apple ID account. This looks like an honest hack on iMessage!? The iPhone doesn't appear to be compromised. It's running iOS 10.0.1 . No new apps. Nothing broken. The Apple ID is secure... What on earth....