That sounds like classic crosstalk and I don't believe there's anything you can do to eliminate it, except to replace your wiring with twisted cable (i.e. Cat 5/6/etc..) or to switch to a wireless phone for at least 1 of your lines so you're not doubling up on the old phone cable. Perhaps others here will have additional suggestions. The problem, as I understand it, is that because the wires in old phone cable run in parallel, the audio can leak from one line to the other. Twisted wire, for whatever reason, eliminates this.