For now, CF is on the TXU EDACS Narrowband system
here. The Cedar Hill site is probably your best chance, but it's not likely you'll hear the Dallas choppers there unless they're in the Cedar Hill area. Rumors are that they will not be on the TXU system much longer, however.
CF also has some conventional freqs, but it's my understanding that they're not used much at this time. Sometimes, you will hear the helos relay patient information on one of the common
Med channels as well.
Dustin