If your only goal is to stop making phone calls I think the man in the middle attack is overkill.
My DirecTiVo is in a camper so it is a major pain to call in every month. I have a lifetime sub so there is no need to call in.
My guess is that all you need to do is to go into the TiVo and tell it that its last successful call was less than a month ago. That would be a lot easier than trying to spoof a phone call with an external computer.
I'm not going to mess around with my TiVo until I can get a small backup of it, but I would be very interested in hearing about any progress you make.
-GhostInTheMachine
