I was a regular user of second line until I left Orange last year. I have no definite knowledge, but my feeling was that the VOIP service had its own "channel" and perhaps accounted for why my top speeds were always a margin below the maximum attainable for my line. As I was on an "old" package I had a local 020 number (restricted choice, but it could be changed) and the free calls to 30 countries option (an earlier version had 100). My usual speeds were in the 4-5Mbps range and it worked perfectly, even for office calls lasting well over an hour. It had better call quality than the analogue line. It didn't become a problem except when my speeds deteriorated for a time to around 2Mbps, when calls broke up. The key point was you always had to use it to keep it.
The other option I still use with an Orange mobile is their (poorly advertised) UMA, it only works with a select few phones but with any ISP, and routes mobile calls over the internet through your router as Home Phone (free calls not out of your allowance). I think it also works for incoming calls, but at its best it's much clearer when the mobile signal is poor.
Edited by deleted (Sat 10-Mar-12 16:24:56)