That's a cool thing to do with a Netduino!
I've also wanted to do something with telephony. I got a GSM board but haven't found any target yet :-)
Have you taken any particular measures for reliabilty and/or security? I mean like having a watchdog reset the board once every hour or something to prevent it from going OFF unintentionally.
i haven't taken any measure for reliability, i tested all week and it seem stable enough. for reset the board at last i can use a SOCKET TIMER. it would be nice if the reset software were implemented on the board.