[ Wire taps are now digital connections. All ‘landline’ calls are actually IP calls once in the network. ]
But for those you need “a paper trail”
An illegal wiretap using “off the shelf components” could be built cheaply and if found there is a cold day in hell’s chance of tracing it.
If I were building such a thing I would use off the shelf components and use the voltage of the phone system to “trickle charge” it.
You’d have to do the tap in the last mile, after the VRAD / DSLAM (that green cabinet on your corner. All the rest is digital.