POTS often had 36V on it to power the ringer on phones so you could potentially up the power transmission by running your system at 14V. Some devices which consume a fixed amount of power would use less current, other devices would just use more power at higher voltage.
[0] https://en.wikipedia.org/wiki/Plain_old_telephone_service [1] http://www.powerstream.com/Wire_Size.htm
But AWG24 cable also has a resistance of 0.08Ω/m (0.024Ω/ft). With a 10m (33ft) cable this amounts to (current has to travel both ways): R=2⋅ℓ=1.6 Ohms, at 3.5A this drops 5.6V. So, in the end you'd loose RI²=19.6W in the cable and only get 22.4W (3.5A at 6.4V) in the load...
You could charge a phone, but even my small laptop chargers are over 60W. The big ones are 90W.
Also don't try this in your house! pushing 3.5A through that little pinner wire it could get hot and start a fire.
But i forgot about the resistance from the load back to the source so above where I said 6m should actually be 3m. Or if two wires are parallel both from source to load and load back to source then we're back to 6m.
You can draw a tiny current without triggering the "off hook" detector, but that is miniscule.