I actually do this here. You only need to break one wire, so only 2 contacts are necessary.
That being said, I have 2 phones in the house THAT ARE NOT SWITCHED. Would you really want to trust X10 to a control primary communication system in the event of an emergency?
I have a peeve with calls after 2100 hours, so most of the phones get switched off, but you never know when it's an emergency. Most people know not to call late in the evening.