We have just set up our new mobile numbers on the ordering system. In theory they should be working from BT lines tomorrow. They are not working from everywhere though and that could take weeks or even months. Inbound SMS is likely to be an issue for a while as well.
However, you can now buy your mobile numbers. They start 074411. They are the same price even though not fully working yet - you are just paying for picking numbers earlier :-)
They must be associated with a voice SIM to work. That does not stop voicemail or diverts though. We have asked OFCOM to confirm this is acceptable usage (as the rules technically do not even allow voicemail!).
The main advantage is inbound calls to your voice SIM are free to receive if the call is to the mobile number as the caller is paying mobile rates to make the call. A&A to A&A calls to A&A mobile numbers are 2.5p/min+VAT rather than free and still the recipient does not pay in those cases. I hope that make sense.
Inbound texts will still cost, sorry - working on that one.
And well done one person ordering two of these numbers before I announced anything!