My BB certainly doesn't talk to the network when I'm typing an SMS reply,
Oh really!
About BlackBerry Messenger
With BlackBerry Messenger (BBM), you can chat and share in real-time with your BlackBerry contacts. For example, you can see when someone has read your message and when the person is typing a reply.
http://docs.blackberry.com/en
Having said that, the AAIB report only identified the last received text ("Yes it’s fine still here") as not having been read, unlike all the prior ones, including a couple that simply said "OK" and would therefore not have needed to be opened if a smartphone was being used, but were.
http://www.aaib.gov.uk/cms_resources...3%20G-CRST.pdf
The first OK was from Witness A to pilot.
In order to reply to that message, which was the next text in the sequence, that OK msg would have been opened as it was part of the conversation.
The second Ok, also from Witness A to pilot, ended that particular conversation as an acknowledgment, so would not necessarily, as you say, have needed to be opened to do just that. However looking at the timeline, a couple of texts were sent and read in that minute. Perhaps there was going to be a reply to that Ok and so it was opened, indicating a read, however the reply msg to the client took priority. Or perhaps while the reply to the client was being typed, the ok msg came through and was quickly looked at during the reply to the client. After all, when typing a msg and another msg comes in, the indication would only be that a new message has arrived and not a shortened text version.
(I wonder if the final msg from the Client - 'Battersea is open', could have been interpreted as a suggestion?)
Anyway, as much as an understanding of text messaging, SMS, messaging, Blackberry messenger etc etc can explain a possible course of events here, I'm sure that the AAIB will have the 'authority' to get whatever information is needed to interrogate the different pings and pongs and timelines from the various networks and devices.