As long as you use a high impedance audio transformer coupled to the line via a couple of 200V+ non-polar caps, there should be no change detectable on the line.
That reminds me, there used to be an old number you could call... I think it was from panel-office served phone lines, what you could call and it would drop your line from the exchange for a minute or so. Great to test for phantom devices attached to your line (line-to-line resistances and capacitance changes)