its always a good idea to use you mobile for outgoing and your landline for incoming, cos you have the mobile anyhow, and you can use your free minutes to call out for clients etc but keep the landline for incoming, almost every single provider of telephone services is linked to bt unless they have their own network ie, virgin, you will find that no matter what you will have to pay the usual bt quarterly line rental, as there i no escape.
have you considered internet telephoney through a computer ? it doesnt cost any money then for you calls ! it uses the exisiting line, if you are online so its no additional cost.