You may have to get a BT voice line after allWhat is Local Loop Unbundling?
Local Loop Unbundling (LLU) enables operators to connect directly to the consumer via BT's copper local loops and then add their own equipment to offer broadband and other services. This process involves operators accessing BT’s local exchange buildings to connect to BT’s network of copper lines which connect them to homes and businesses.
There are two types of unbundled line:
* A fully unbundled line gives operators the exclusive use of the copper line.
* A shared access line only gives operators the use of the high frequency channel used for broadband and will also be used by the customer's fixed-line voice provider.
Ofcom believes that development of the LLU market, to allow operators to target infrastructure investment and to develop scale in the creation of high-speed data services, will be critical in ensuring a fully competitive and innovative telecoms market for the long term.

Hopefully o2 can just unbundle the broadband side of things. After your contract is done with BT you could move to a fully unbundled solution from o2 if it saved you some money.
Either way BT are getting your money
Either you pay them directly or the ISP has to pay them. Even for fully unbundled solutions I believe BT gets paid for the use of their copper lines.Sadly, BT can charge what they like for re-connecting your line, and looks like Offcom have decided their £125 charge is reasonable.
