@pedrwrote:Why are BT billing for line rental then?
This arrangement was set up many years ago, possibly before LLU (and with Tiscali before it became Talk Talk).
Solus line and/or carrier pre-select would be my guess.
Solus line - you pay line rental to BT, you pay TalkTalk for the broadband
Carrier pre-select - you pay line rental to BT, you pay another for the call usage
Both were quite common years back, gradually phased out now and no longer a product sold because providing support was an issue.
Keeping the number maybe dependant on who provided it originally.
If it's NOT BT it could be lost in the transfer to another provider.
For a more technical/overall reply to your question you may want to try Broadband Forums :: think broadband they provide independent advice.