I didn't say you couldn't, I said I wouldn't unless it's a dedicated circuit. IP is funny, some times the packets don't get there in the same order they left. That's whats great about IP, there is always an alternate route, but with that you can get packets out of order, or not have a high enough priority that they can sometimes get left behind. Your computer doesn't care, it may load different parts of a web page not in the order, but audio has to arrive in the correct order and at the right time. Specific setting in the routers must be used to prevent problems like that.
As far as after the local exchange, it's not IP but probably T-1 or T-3 circuit, a whole different animal. For those agencies using them successfully, that's good, I am just not all that confident with it after problems with some VOIP circuits we have. Maybe the newer stuff address that.
Also it's funny how most everyone in Public Safety now doesn't consider audio delays as a problem, as it was first thought of in the 80's. Some of the early rolling code scramblers had a 1/2 sec delay, which was not considered acceptable at that time. Now a days with all the digital stuff 1 sec is common.