I don’t think you’re really asking whether wiretaps still exist… are you?

Imagine a particular scenario: you come to me for confession (ok, yeah – I just ordained myself for the sake of this example…

) and I allow you to use your cell phone. Unbeknownst to either of us, the FBI has a warrant to tap your cell phone, and sure enough, they’re listening in on our “confession” conversation. When you confess to running numbers, selling drugs, murdering people, and doing whatever else, the FBI now has your confession on tape. And, in that context, I will have participated in the “outing” of your confession with the authorities. (Looks like I’m not going to continue to be doing priestly ministry very long!). And worse, I could be called to testify in your trial. I might refuse to relate what you said, but they don’t need me to do so; all they have to ask is “Fr Gorgias, did you have a phone conversation HomeschoolDad on 12/18 at 4pm? Was the context of the conversation a celebration of the Catholic Sacrament of Reconciliation?”
And… there you go. A really, really good reason not to use cell phones for confessions.