In a post I wrote a while ago, I talked about a way to make a TiVo make phone calls over a VoIP connect. Vonage used to have instructions on dial prefixes that would work. (It looks like they removed all of those instructions.)
I just got a new TiVo, and was trying to set it up, and the second phone call was failing. I did a search on the web, and I found these instructions that would allow the TiVo to make the phone calls over a broadband connection.