NTLworld certainly don't require a second phone line.
I use it on a single BT phone line.
Initial setup involves purchasing a 10 quid box to go between your phone & the phone line.
This diverts your calls through the NTL network. You can either plug your PC into it with a double adapter for the phone, or only plug your phone into the box & then set up your dialing properties to pre-dial a code that does the same thing.
Apart from that the minimum charge is 14.99 quid / month. This provides the same value of local/national phone calls. Phone calls over this value, or not part of the included calls eg directory inquiries, some premium rate calls, overseas etc, are charged extra.
Effectively, internet access is free.