The original Triplett Fox & Hound was an audio signal generator (2KHz I believe) and an audio amplifier with an inductive pickup - I don't recall them having any fancy filtering.
An ISP using copper lines - are we talking co-ax, twisted pair or good old fashioned POTs?
The old POTs system uses 48v DC signalling so anything connected to it will need to withstand that voltage, for that reason I don't see 2 x 9v batteries as being a potential issue and I don't see differential signalling as being effective for an inductive type pickup as used with most tone tools - the concept behind coax and twisted pair is that both technologies reduce "noise pickup", the corollary of which is "signal radiation" which is, so to speak, equally affected.
I don't mean to discourage you, but I've used tone tools from several manufacturers, including the original Triplett Fox & Hound as well as a couple of units from Paladin & Fluke specifically designed for CAT 5 UTP and not one of them was worth a damn in a "live" environment, not even the fancy Fluke Intellitone with digital signalling.