Yeah, Bryan clarified it nicely, and yes, what I was talking about was "the same phone line and number appearing at 2 separate premise locations in the same wire center serving area", which is what *I've* always seen called an OPX. They generally just set it up as a separate service, and then multiple them together at the MDF; both ring simultaneously, and if both ends pick up, all parties can speak.
But, given the way it's constructed, obviously it's only useful for the LECs circuits.
I still think the OP would be best off using wifi to solve his actual problem... :-)
