Remember that recording of Newt Gingrich that made its way to Rep. Jim McGermmot back in '96 I think it was?
Some elderly Florida couple named the Martins just happened to be listening to their police scanner when it picked up Newt's phone conversation and they just happened to have a recording device in their vehicle!
Strictly speaking, Newt wasn't being wiretapped, but he was definitely being monitored or surveilled.
I suspect the same thing happened with Flynn and the Trump Tower.
You don't necessarily have to "tap" any wires to listen in on a cell phone conversation.
That was John Boehner, not Newt.