British Telecom don't allow you to have their incoming call only service if you have ASDL broadband with them, and neither are you allowed it if you use BT Mobile for your mobile phone?
I gathered that from the fact you don't actually dial anything. But if its a line designed to let you make out going calls, why should they let you make outgoing ADSL connections either
adsl isn't magic, it still needs to connect and synchronise with the exchange before it works much like your phone needs a dial tone from the exchange before it can attempt to make a voice call.
I can see it becoming very popular very quickly if it were allowed - make all your outgoing voice calls over broadband using VOIP, and get all the incoming ones through your standard phone line with cheap rental. Perfect.
I think every other telco would have the same condition, personally.