Well you could put a wireless card on some sort of ip enabled butt set and you can tap the actual wire. Hopefully you have some sort of dsps in this magical butt set that can sample and send out the voice and for simplicity sake just have it stream the traffic to something like netmeeting …assuming your wireless lan hub is close enough.
Thats one way, I am sure there are prettier ways, but its just something i had in mind.
BTW…you will always need access to the copper or circuit that the person is using…unless they are using voip….then you can use some of the newer CALEA/LI technologies coming out for IP. 🙂