Off the top of my head I can't think of why that would be. You still need a camera/microphone at each end. No difference from the surveilance standpoint. Digital has lots of advantages, but I don't think that's one of them.
Well, I'm not an ex-telecoms guy. I'm a CURRENT telecoms guy. I engineer cellular networks.
Yeah, and I've heard some doozies while out troubleshooting the network. Some funny, some disgusting. Trust me, folks, you REALLY don't want to listen in to a couple of South Central (L.A.) whores talking business. 
OTOH, I caught my boss skipping out early a couple of times. Too bad I couldn't record it.
The digital phones.... uh, which technology? It DOES make a difference. Anyway, whether GSM or CDMA there is some encryption inherent to the technolgy. GSM offers some extra encryption which is limited as stated. But it's not trivial to crack a GSM call - it can be done in the lab but you'd need a lot of luck and the capabilities of a major gummit spy lab to do the job out in the real world. With CDMA, forget it. In fact, an encryption option was built into the CDMA standard, but nobody ever bothered to implement it. With inherent security beyond the capability of anybody other than the NSA to break it, why bother to add more encryption? Who would pay for it?
But so what? With a warrant, they just tap in at the central switch. <shrug> The FBI was allllll bent out of shape at the idea of CDMA being used in civilian applications, until that fact dawned on them.
