I moved the line that supports my ADSL to Zen so that they can manage the lot (this meant that they could get an Openreach engineer round to replace the socket for me)
Basically the same service you are used to with your broadband for your phone service.
Only thing I would say and I fed this back to them in a survey on the product is that 0845/0870 calls aren't included in the inclusive minutes on Talk Plus whereas BT and others do include these now, if Zen were to add this I maybe tempted to move my other line to them too.
Virgin (ADSL) => Namesco => Newnet => O2 => Plusnet => Zen => Newnet => Zen Lite 8000
Note: I don't lay turf for anyone. astro or otherwise, all views and opinions expressed are my own based on experience.