Sorry! It's a PSR 500??
If it's like the Radio Shack equivalent models, when you are scanning the trunked system, hit manual, press the button under TSYS, press the analyze button, and then use up and down to find 859.7125. Make sure you hear the control channel, and look at the percentage that flashes on the display. A good decode is like 99%. If it's low or fluctuating a lot, you need to try moving your scanner around or try a yagi antenna.