First of all, there is no reasonable expectation of privacy in inmate/confinee calls. The system they use plays a message to that effect at the beginning of the call. Jail/prison/brig phone calls are routinely recorded, and prosecutors listen to those phone calls. That's why defense attorneys will come see you in person. It's also why every once in a while you'll get an admission or other indications of guilt from someone who doesn't listen to those warnings.

http://www.cnn.com/2011/CRIME/03/26/jailhouse.calls.recordin...

Second, an anecdote. The system they use in my jurisdiction uses phone cards. We once had a group of confinees switch phone cards to make it difficult to trace who was speaking (and at the very least introduce reasonable doubt). I listened to dozens of hours of audio in one case, just hoping for the caller to identify himself (since the advent of caller ID, it seems a lot of people just say "Hey"). Voice printing would have been very helpful here.