The sales office need a business line. Don't forget they need that line to sell houses! Besides that, they would have paid an extortionate amount to BT openreach to get that work done with the underground distribution points and the ducting put in place so it's ready for cabling. Therefore, BT would have treated the developers as highest priority for a number of lines. If you think about how much business they must put BT's way.
I say that the chairman's office would help more than other department, you will get to speak to someone that talks sense in english but i got in touch with them first by email before they made a call about that email the same evening. In my case with them, things have moved quicker but I still often feel like i'm being given incorrect information with them sometimes but they tend to blame the planner, contractor or openreach. They have known about the situation with my order since the 8th april, so that's more than 2 months they have had the complaint for.
In the Chairman's office, it will be one person that will deal with your order, unless they are not there and another will check progress for you.
Obviously, I have the number for the chairman's office and the desk number that gets me straight through to the person that deals with my order. So if you get someone in the office to take charge of the order, i would suggest asking that person for their direct number that goes to their desk. It's better than dialling the 0800 number, as I have to call from a mobile. 01 numbers included in my allowance, 08 numbers cost 2p per minute, though if you go to a payphone, they a free to call from there. Maybe go to the developers sales office and ask why its taking so long, ask if you can call BT from their phone. They will probably be annoyed by this But might complain to BT for you.
Maybe if you had ordered a business line, it may have speeded things up. I hear they can claim good compensation money from BT if they are waiting too long. I'm not totally sure if that happens with orders but I know it does if the business line has a fault or something.
Why are you waiting for BT to bill you? I'd cancel any direct debit you might have set up with them, they are not the easiest company to claim money back from.