Chris

If it were possible to have this service without paying line rental to BT then it would truly be the holy grail of telecoms. But the very least you have to pay to BT is, IIRC, £10.50 a month, which includes all your off-peak UK landline calls charged at 5.5p per hour (merely the connection charge and a need to redial once every 60 minutes).

So, your minimum monthly charge for this service is really £20.49, which is admittedly cheaper than the equivalent BT offering, 'BT Together Option 3', which gives you all landline calls, at all times, for no extra charge (but redial every hour) for £25.50 a month.