I don't know if the Virgin deal is what is called SMPF where the phone line is still with BT Wholesale but you pay rental and calls to Virgin, or FPMF where your phone line actually gets connected into a Virgin phone system. (That's Shared or Full Metallic Path facility).

Examples of the first are Plusnet and Madasfish. Examples of the second are TalkTalk and at least some of the Tiscali connections.

If it is the second type you need to get a Return to Donor actioned to get it back to BT Retail - I think you do that through BT. If it is the first it would depend on whether or not Virgin would let you have another ISP sharing the line - I doubt it. I must admit I'm a little out of my depth on this one.

Your line at the moment will be connected to a Virgin DSLAM, whatever the position regarding the phone service. With O2 Access it will be connected to a BT Wholesale DSLAM. (When you are on broadband your line is connected to the broadband equipment and the phone aspect of it is then connected from the DSLAM to the phone service supplier. Whether BT Wholesale let other phone suppliers connect to their DSLAM I don't know).

I suggest if no-one with more knowledge than me comes in with answers to this bit fairly soon that you open a thread in general Broadband Chatter on just this question. Or to save time, do that now and then post a link to it as a reply to this post asking anyone here to reply there on this subject - so all the answers are together and people don't waste their time in one thread telling you what someone else has already said in the other . There is nothing more annoying.