Thanks for posting. I'm sorry for the delay in getting back to you. I've asked around and from what I can gather it's not possible to transfer the calls sign number to a seperate phone line.
BT people like to "pretend" it cannot be done. In reality I have transferred a primary number to a Call Sign number in the past, and I have also transferred the digital number of an Home Highway "ISDN" line to become a Call Sign number too. They just don't want to bother to say they can do it because there's probably nothing in it for them and on some exchanges it might be more complicated than on others.
The last time I did it I had an argument with the man on the phone, who told me it could never be done until I exampled the details of when I'd done it before. It was then done.
Funnily enough I want to move a Call Sign number from one line to another right now, and then switch it with the primary number on the other line, so the existing primary number of the other line becomes the Call Sign. I'm getting the "cannot" be done answer at the moment, so I'll shortly be writing a letter to BT exampling how it can be done given I've had it done twice before.
Remember BT don't work for you, you work for them! How dare you even suggest they do something that you might find useful.
I have also been using call sign for a few years and, like you, wish to transfer that number to a separate line. Did you ever find out if this is possible?
I cannot seam to find the link to the the 'solved' icon and also unable to find any information about this anywhere on the BT website.
Hoping someone out there will be able to assist.