also interest in such an app. At moment my solution is to block all incoming calls and texts to the secondary number but of course callers dont know there texts are blocked, they think its been sent as normal, but my silence normally gives them a hint something has gone wrong somewhere's. However they do get a message if they make a call.
Networks are already counteracting this in the UK with vodafone minimum call rate now 15p and 02 to go to 20p minimum call charge. So in the end like always the customer lose loses.