Well I went into a local MS Store today and put my AT&T sim into one of their Lumia 640 phones running 8.1 update 2. Paired my band up to that phone. Asked one of the store employees to send me a text message and everything worked fine when I dictated a response via Cortana. So that pretty much nails it for me...hmm might be dropping back down to 8.1 official from this 10.0.10512.1000 version 
