That will work great, until someone cuts the phone and cable lines to your house and your modem goes dead.
When I used to work on security systems we would bring along a shovel for installs. We would bury the phone line and move the phone box inside to the basement. Made it inconvenient for the home owners if they needed changes to their service (they would have to be home to let the phone guy in), but gave them a heck of a lot more security.
For my own place I put up a dummy box with some wires running into it. If they cut the wire it set off the alarm.